[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Thanks to David Edelsohn I have AIX access now. The issue reported
by Pablo is the same as #41540, for a summary see msg375480.

It is a trivial issue that requires that ulimits are in place due to
the fact that AIX over-allocates petabytes even when the physical memory
is just 16GB.


I have verified that the Python-3.7.7 release, which contains the
feature, behaves exactly like master:


No limits
=

./configure CC=xlc AR="ar -X64" CFLAGS="-q64 -qmaxmem=7 -qlanglvl=extc99 
-qcpluscmt -qkeyword=inline -qalias=ansi -qthreaded -D_THREAD_SAFE 
-D__VACPP_MULTI__" LDFLAGS="-L/usr/lib64 -q64"

skrah@gcc119:[/home/skrah/Python-3.7.7]./python -m test -uall test_decimal
0:00:00 Run tests sequentially
0:00:00 [1/1] test_decimal
Killed


There is no segfault, the program receives SIGKILL.



Data limits (-bmaxdata)
===

./configure CC=xlc AR="ar -X64" CFLAGS="-q64 -qmaxmem=7 -qlanglvl=extc99 
-qcpluscmt -qkeyword=inline -qalias=ansi -qthreaded -D_THREAD_SAFE 
-D__VACPP_MULTI__" LDFLAGS="-L/usr/lib64 -q64 -bmaxdata:0x8"

skrah@gcc119:[/home/skrah/Python-3.7.7]./python -m test -uall test_decimal
0:00:00 Run tests sequentially
0:00:00 [1/1] test_decimal

== Tests result: SUCCESS ==

1 test OK.

Total duration: 18.0 sec
Tests result: SUCCESS



In summary, since 64-bit AIX users should be familiar with data limits,
I don't consider the situation a _decimal bug at all.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

The 3.9 change (see #40874) works successfully on all buildbots, except for the 
32-bit xlc bot which should use c99_r.

Additionally, it has been tested with the latest gcc/clang/icc/cl.exe, static 
analyzers and clang-tidy.

It survives brute force allocation failure tests under Valgrind and all 
sanitizers (ASAN + integrated LSAN, UBSAN, TSAN).


Except for one (standalone LSAN), and that is an LSAN issue:

https://github.com/google/sanitizers/issues/1257


It looks very similar to the xlc issue and was found by using a debugger, which 
is apparently not possible with the xlc toolchain. :-)

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 0f5a28f834bdac2da8a04597dc0fc5b71e50da9d by Stefan Krah in branch 
'3.8':
[3.8] Revert bpo-39576: Prevent memory error for overly optimistic precisions 
(GH-20747)
https://github.com/python/cpython/commit/0f5a28f834bdac2da8a04597dc0fc5b71e50da9d


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 22faf6ad3bcc0ae478a9a3e2d8e35888d88d6ce8 by Stefan Krah in branch 
'3.7':
[3.7] Revert bpo-39576: Prevent memory error for overly optimistic precisions 
(GH-20748)
https://github.com/python/cpython/commit/22faf6ad3bcc0ae478a9a3e2d8e35888d88d6ce8


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 9bd891920a5186b7d02281ea9966225efa0ceba1 by Stefan Krah in branch 
'3.7':
[3.7] Revert bpo-39576: docs: set context for decimal arbitrary precision 
arithmetic (GH-20746)
https://github.com/python/cpython/commit/9bd891920a5186b7d02281ea9966225efa0ceba1


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 32c1fb07e6f2ded90e5dd24d4b46b7aa7a795d2e by Stefan Krah in branch 
'3.8':
[3.8] Revert bpo-39576: docs: set context for decimal arbitrary precision 
arithmetic (GH-20745)
https://github.com/python/cpython/commit/32c1fb07e6f2ded90e5dd24d4b46b7aa7a795d2e


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset c0b79450bc9e93105799528151c48d25af8240a3 by Stefan Krah in branch 
'3.7':
[3.7] Revert bpo-39576: Clarify the word size for the 32-bit build. (GH-20744)
https://github.com/python/cpython/commit/c0b79450bc9e93105799528151c48d25af8240a3


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 706de4e5a4b21880c67f6b90e3a2147a258d6fc5 by Stefan Krah in branch 
'3.8':
[3.8] Revert bpo-39576: Clarify the word size for the 32-bit build. (GH-20743)
https://github.com/python/cpython/commit/706de4e5a4b21880c67f6b90e3a2147a258d6fc5


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

I think I'm going to revert this for 3.7 and 3.8 -- not because of xlc (it is 
almost certainly a compiler or missing flag error), but because coordination 
with the Linux distributions is a mess, see #40874.

I really want the system libmpdec to be the same as the one in Python.

So, there has been one 3.7 and 3.8 release in the meantime. However, I highly 
doubt that anyone will have relied on this in the meantime.

In the worst case, people can compile 3.7/3.8 with the upcoming libmpdec-2.5.0.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> Traceback, does it work --without-pymalloc?

No, it does also not work with `--without-pymalloc?`.

> Has anything emerged xlc-wise?

Not for now, I keep investigating and I may try to contact
IBM about this, but at this stage, I am getting more confident
that this is a bug in xlc or something going on with the platform
or the libC implementation.

I am currently chasing something weird that I saw while investigating
regarding too big malloc() calls and the data segment size on AIX that
produces a similar hang.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Has anything emerged xlc-wise? Traceback, does it work --without-pymalloc?

I have the feeling that (in many OSS projects) the more complex xlc
issues never get resolved after the initial report.

So I'm contemplating to do the same as 

   
https://devhub.vr.rwth-aachen.de/soehrl/arbor/commit/775fe80796c717da4ed61e1cb7ace27268afc965

and disable the xlc build for _decimal.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

The lower MAX_PREC for 32-bit could be the reason.

On the other hand, historically, suncc and xlc have had a lot of
problems with the 64-bit build. The winner is suncc, which seriously
miscompiled libmpdec without a whole litany of flags:

https://bugs.python.org/issue15963#msg170661


This is why I keep insisting on experimenting with obscure flags,
but of course the actual cause may be different.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[STINNER Victor]

Change by STINNER Victor :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

I will try to investigate that but it think this is because without 
OBJECT_MODE=64 the value of decimal.MAX_PREC is much lower (42500
instead 99) so whatever is happening downstream with the higher 
does not happen with the lower one.

I will update If I found something useful on what is going on.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Since xlc has elementary bugs like

   https://github.com/openssl/openssl/issues/10624

it may be worth checking out if this is an optimizer bug with -q64.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

After some debugging, I discovered that the new test succeeds if I configure 
and compile CPython without 'OBJECT_MODE=64' set.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Sorry, I'm reacting like Granlund now and close. These discussions
lead nowhere.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

Just to be clear: I am saying that the *exact* regression is manifested with 
the new test because that is the behavioural difference that I experienced and 
how I found this problem. 

> but just ran the 3.7.6 tests that do not include any tests with MAX_PREC.

Independently on other considerations, If the test suite fails in 3.7.7 and 
succeeds in 3.7.6 that is a regression.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> In other words, contrary to your earlier dismissal, you did NOT
run _decimal on AIX with MAX_PREC but just ran the 3.7.6 tests
that do not include any tests with MAX_PREC.

I did, and it raises MemoryError:

❯ uname -a
AIX 1 7 powerpc 00CCAD974C00 AIX

❯ python3.7 --version
Python 3.7.6

❯ python3.7 Lib/test/test_decimal.py

...

==
ERROR: test_maxcontext_exact_arith (__main__.CWhitebox)
--
Traceback (most recent call last):
  File "Lib/test/test_decimal.py", line 5506, in test_maxcontext_exact_arith
self.assertEqual(Decimal(4).sqrt(), 2)
MemoryError

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

"The **exact** regression is that I could run the test suite without any crash 
or freeze on this AIX system
with 3.7.6 and I cannot in 3.7.7. At the very least this is due to the fact 
that there is a new test that crashes/hangs."


In other words, contrary to your earlier dismissal, you did NOT
run _decimal on AIX with MAX_PREC but just ran the 3.7.6 tests
that do not include any tests with MAX_PREC.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> Acknowledge that gcc builds work on the AIX buildbots (a fact that has been 
> entirely ignored so far).

I do acknowledge that. I am not saying that I am sure there is a bug in the 
code. For what I know at this point
it may be something with xlc, with the OS itself, with the libc version or 
something misconfigured on the machine.

The only thing I know is that before I could run the test suite without any 
problem on 3.7.6 and now I cannot
in 3.7.7, and because 3.7 a maintenance branch I though this is a possible 
regression, so I reported it.

> are you using xlc or xlc_r?

I am using xlc_r. I am saying xlc because that is the name of the compiler. The 
_r suffix activates thread-safe compilation (which
I am doing). Apologies for the confusion.

> State the exact regression

The **exact** regression is that I could run the test suite without any crash 
or freeze on this AIX system
with 3.7.6 and I cannot in 3.7.7. At the very least this is due to the fact 
that there is a new test that crashes/hangs.
Why this happens I still have no idea as I am currently investigating. 
Debugging on AIX is not a pleasant task.


> n short, given the flakiness of the xlc toolchain I'm not even
sure if anything can be classified as a regression. Most xlc users
I've talked to have switched to gcc.

I notified the behaviour different here so we can discuss about this as a team. 
If we decide that this is xlc's fault or
is not worth to investigate or that because AIX is not really fully supported 
we do not want to spend resources I can totally
understand it. I thought this was important enough because this change was in 
3.7, which is supposed to be stable across minor
releases (the same applies for 3.8 if the problem also appears there).

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

It is also instructive how Granlund views xlc:

https://gmplib.org/list-archives/gmp-bugs/2010-November/002119.html

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

A focused issue report would include at least the following:

  1) Acknowledge that gcc builds work on the AIX buildbots (a
 fact that has been entirely ignored so far).

  2) State the exact regression: msg364373 (which was also ignored,
 are you using xlc or xlc_r?) says that there have always been
 problems with xlc unless configured in a very specific manner.

 Then obviously we need the exact Python code that worked in
 3.7.6 but not in 3.7.7.


In short, given the flakiness of the xlc toolchain I'm not even
sure if anything can be classified as a regression. Most xlc users
I've talked to have switched to gcc.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


--
assignee:  -> skrah
keywords:  -3.7regression, patch

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> I mean, it's not okay to complain about a regression and then
mention xlc about 10 mails later.

How is this related? Or is not ok to report a behaviour change in a stable 
release even if the platform is "best-effort"?

A regression by definition is a behaviour change between two versions. AIX is a 
"best-effort" platform, not a fully-supported one and I am well aware. I have 
not asked for a revert in anycase, precisely because of that (that decision 
should be up the release manager anyway), I am just communicating the 
regression. And doing so is perfectly "ok", even If I am using xlc (which is 
the native compiler on AIX).

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

BTW, if you are compiling with xlc and there"s no xlc buildbot,
perhaps a company-that-shall-not-be-named should provide one.

I mean, it's not okay to complain about a regression and then
mention xlc about 10 mails later.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

Btw, this is AIX 7.1.0.0 with xlc in case that is relevant.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

These flags worked for xlc when snakebite was still up:

./configure CC=xlc_r AR="ar -X64" CFLAGS="-q64 -qmaxmem=7" LDFLAGS="-q64"

-qmaxmem was always finicky, I remember segfaults too (AIX problem,
not libmpdec problem).

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> If you are fortunate enough to have access to an AIX system, I guess
you have to find out why POWER6 AIX 3.8 and PPC64 AIX 3.8 apparently
work on https://buildbot.python.org/ but your 3.7 does not.

I am working on trying to debug where the problem comes from, but meanwhile, I 
wanted to report here that this introduced a difference between Python 3.7.6 
and Python 3.7.7

I still did not test Python 3.8 but I suppose that it has the same problem.

> Are you compiling --with-system-libmpdec?

No, I am compiling with:

CFLAGS+= -qmaxmem=-1
OBJECT_MODE=64
CONFIGURE_ARGS=--enable-shared --with-system-ffi --without-computed-gotos 
ac_cv_rshift_extends_sign=no

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Hi Michael, in case you have built 3.7.7 on AIX, have you observed
any problems with test_decimal?

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

> Well, I just did and I can confirm that reverting the 3.7 backport fixes the 
> problem.

If you are fortunate enough to have access to an AIX system, I guess
you have to find out why POWER6 AIX 3.8 and PPC64 AIX 3.8 apparently
work on https://buildbot.python.org/ but your 3.7 does not.

It seems rather strange to me. Are you compiling --with-system-libmpdec?

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> Thanks for the lecture. This is an esoteric case between bugfix and
feature that only occurs with very large context precisions.

Nobody is lecturing anyone. I am just asking why this was backported.

> If Bloomberg isn't happy with _decimal, they can stop using it.
I'm not paid by them.

Stefan, nobody has mentioned Bloomberg in this issue so please, stop.


> Also this is a rather bold statement since probably no one has ever
run _decimal on AIX with MAX_PREC.

Well, I just did and I can confirm that reverting the 3.7 backport fixes the 
problem.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

> This code has introduced a regression in AIX in Python 3.7.7

Also this is a rather bold statement since probably no one has ever
run _decimal on AIX with MAX_PREC.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

> This looks like a "new feature/improvement". Why was this code backported to 
> a stable version?

Thanks for the lecture. This is an esoteric case between bugfix and
feature that only occurs with very large context precisions.

If Bloomberg isn't happy with _decimal, they can stop using it.
I'm not paid by them.

Which AIX buildbot fails? There are many green ones.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Change by Pablo Galindo Salgado :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

This looks like a "new feature/improvement". Why was this code backported to a 
stable version?

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

This code has introduced a regression in AIX in Python 3.7.7 as the new 
"test_maxcontext_exact_arith" test hangs indefinitely or just segfaults.

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nosy: +pablogsal
resolution: fixed -> 
status: closed -> open

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset c6ecd9c14081a787959e13df33e250102a658154 by Miss Islington (bot) 
in branch '3.8':
bpo-39576: Clarify the word size for the 32-bit build. (GH-18616) (#18618)
https://github.com/python/cpython/commit/c6ecd9c14081a787959e13df33e250102a658154


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 24c570bbb82a7cb70576c253a73390accfa7ed78 by Miss Islington (bot) 
in branch '3.7':
bpo-39576: Clarify the word size for the 32-bit build. (GH-18616) (#18617)
https://github.com/python/cpython/commit/24c570bbb82a7cb70576c253a73390accfa7ed78


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[miss-islington]

Change by miss-islington :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset b76518d43fb82ed9e5d27025d18c90a23d525c90 by Stefan Krah in branch 
'master':
bpo-39576: Clarify the word size for the 32-bit build. (#18616)
https://github.com/python/cpython/commit/b76518d43fb82ed9e5d27025d18c90a23d525c90


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[miss-islington]

Change by miss-islington :


--
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nosy_count: 5.0 -> 6.0
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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Tim Peters]

Tim Peters  added the comment:

Thanks, Stefan!  This turned out better than I expected ;-)

I'm closing the report, under the likely assumption that nobody cares enough 
about obscure inelegancies in the Python version to bother.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

libmpdec and the docs are done, the question remains what to do with
decimal.py.

It has the same behavior, but I don't think users are running
decimal.py with very large precisions.


Anyway, unassigning myself in case anyone else wants to work on a patch.

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resolution:  -> fixed
versions: +Python 3.7

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset d6965ff026f35498e554bc964ef2be8f4d80eb7f by Miss Islington (bot) 
in branch '3.8':
bpo-39576: docs: set context for decimal arbitrary precision arithmetic 
(GH-18594) (#18597)
https://github.com/python/cpython/commit/d6965ff026f35498e554bc964ef2be8f4d80eb7f


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 00e45877e33d32bb61aa13a2033e3bba370bda4d by Miss Islington (bot) 
in branch '3.7':
bpo-39576: docs: set context for decimal arbitrary precision arithmetic 
(GH-18594) (#18596)
https://github.com/python/cpython/commit/00e45877e33d32bb61aa13a2033e3bba370bda4d


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[miss-islington]

Change by miss-islington :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[miss-islington]

Change by miss-islington :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset a025d4ca99fb4c652465368e0b4eb03cf4b316b9 by Stefan Krah in branch 
'master':
bpo-39576: docs: set context for decimal arbitrary precision arithmetic (#18594)
https://github.com/python/cpython/commit/a025d4ca99fb4c652465368e0b4eb03cf4b316b9


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Updated docs:

https://github.com/python/cpython/pull/18594

The PR uses some of Tim's suggestions while also explaining how to
calculate the amount of memory used in a single large decimal.

Hopefully it isn't too much information.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

"So non-integer powers are left out" in isolation would indeed be
wrong, but actual sentence is unambiguously qualified with:

"... since _decimal has no notion of exact non-integer powers yet.",
which clearly states that exact non-integer powers exist and
_decimal does not recognize them (it usually produces exact results
in the respective cases but still sets Inexact).

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Vedran Čačić]

Vedran Čačić  added the comment:

Well, it all depends on how do you intend to read it. To me, the closing quote 
and "So non-integer powers are left out" after it suggested that the 
non-integer powers being left out is somehow a consequence of the above 
paragraph. When in fact instead of "so" I think "except" or "alas":-) should 
have been.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Vedran, msg362365 is meant to say:

"This PR implements $SOMEWHAT_MATHEMATICAL_SPEC except for inexact power."

Had I put the caveat inside the statement as well, the message would have been:

"This PR implements $SOMEWHAT_MATHEMATICAL_SPEC_EXCEPT_FOR_INEXACT_POWER except 
for inexact power."


Bur GNU is not UNIX and all that... ;)

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Vedran Čačić]

Vedran Čačić  added the comment:

Hm... is "exact result" a technical term that's defined somewhere? Because to 
me it seems that this

> "If there exists an exact result at a lower precision, this
result should also be returned at MAX_PREC (without MemoryError)".

is a mathematical statement, and surely for Decimal('0.07776') ** 
Decimal('0.2') _there exists_ an exact result at a lower precision. Maybe this 
should be reworded as "If there exists ... _and an implementation knows about 
it_, it should also be returned ..."

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset b6271025c640c228505dc9f194362a0c2ab81c61 by Miss Islington (bot) 
in branch '3.8':
bpo-39576: Prevent memory error for overly optimistic precisions (GH-18581) 
(#18584)
https://github.com/python/cpython/commit/b6271025c640c228505dc9f194362a0c2ab81c61


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset c6f95543b4832c3f0170179da39bcf99b40a7aa8 by Miss Islington (bot) 
in branch '3.7':
bpo-39576: Prevent memory error for overly optimistic precisions (GH-18581) 
(#18585)
https://github.com/python/cpython/commit/c6f95543b4832c3f0170179da39bcf99b40a7aa8


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[miss-islington]

Change by miss-islington :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[miss-islington]

Change by miss-islington :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:


New changeset 90930e65455f60216f09d175586139242dbba260 by Stefan Krah in branch 
'master':
bpo-39576: Prevent memory error for overly optimistic precisions (GH-18581)
https://github.com/python/cpython/commit/90930e65455f60216f09d175586139242dbba260


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

BTW, this PR implements the invariant:

"If there exists an exact result at a lower precision, this
result should also be returned at MAX_PREC (without MemoryError)".

So non-integer powers are left out, since _decimal has no notion
of exact non-integer powers yet.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

Fortunately libmpdec raises MemoryError almost instantaneously, so the PR 
retries 
the affected operations with estimated upper bounds for exact results without 
slowing down the common case.

The docs still need updating because people will still wonder why 1 / Decimal(3)
freezes the Linux machine at MAX_PREC. ;)


The Python implementation cannot use this approach because memory grows slowly. 
 
I'm not sure though if anyone _would_ run _pydecimal at MAX_PREC.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Change by Stefan Krah :


--
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stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18581

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Tim Peters]

Tim Peters  added the comment:

Formulas based on physical RAM probably work well on Linux, but not so well on 
Windows:  Windows has no "overcommit".  Whether a virtual memory request 
succeeds on Windows depends on how much RAM (+ swap space, if any) has already 
been requested across all processes.  It doesn't matter whether pages have 
actually been created, merely the total number of pages that _may_ be created.

I never bumped into this issue before, because I never used MAX_PREC before ;-) 
 When I intended to do exact arithmetic in `decimal`, I did this instead:

- guesstimated the max number of decimal digits a computation would need

- set `prec` to be comfortably - but not massively - larger than that

- enabled the Inexact trap, so if I guessed too low I'd get an exception

Maybe the docs could suggest that instead?

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

> This isn't purely academic.  The `decimal` docs, at the end:

Yes, that is a good point. I think for libmpdec I'll just do a trial divmod if 
prec > BIGNUM_THRESHOLD.



> Perhaps the only thing to be done is to add words to the part of the docs 
> _recommending_ MAX_PREC, warning about some "unintended consequences" of 
> doing so.

This, too. Probably some formula based on the amount of available RAM would do.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Tim Peters]

Tim Peters  added the comment:

Vedran, as Mark said, the result is defined to have no trailing zeroes.  In 
general the module strives to return results "as if" infinite precision were 
used internally, not to actually _use_ infinite precision internally ;-)  Given 
the same setup, e.g.,

>>> i * decimal.Decimal(0.5)
Decimal('2.0')

works fine.

This isn't purely academic.  The `decimal` docs, at the end:

"""
Q. Is the CPython implementation fast for large numbers?

A. Yes. ...
However, to realize this performance gain, the context needs to be set for 
unrounded calculations.

>>> c = getcontext()
>>> c.prec = MAX_PREC
>>> c.Emax = MAX_EMAX
>>> c.Emin = MIN_EMIN
"""

I suggested this approach to someone on Stackoverflow, who was trying to 
compute and write out the result of a multi-hundred-million-digit integer 
exponentiation.  Which worked fine, and was enormously faster than using 
CPython's bigints.

But then I noticed "trivial" calculations - like the one here - blowing up with 
MemoryError too.  Which made sense for, e.g., 1/7, but not for 1/2.

I haven't looked at the implementation.  I assume it's trying in advance to 
reserve space for a result with MAX_PREC digits.

It's not limited to division; e.g.,

>>> c.sqrt(decimal.Decimal(4))
...
MemoryError

is also surprising.

Perhaps the only thing to be done is to add words to the part of the docs 
_recommending_ MAX_PREC, warning about some "unintended consequences" of doing 
so.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

The feature would be nice to have; however, if you choose the precision to 
match the amount of available RAM things work (I have 8GB here, one word in the 
coefficient has 19 digits for the 4 bit version):

>>> from decimal import *
>>> c = getcontext()
>>> c.prec = 8 * 2**30 // 64 * 19
>>> c.prec
2550136832
>>> i = Decimal(4)
>>> i / 2
Decimal('2')



So I wonder if we really need to do something here.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

MAX_PREC is chosen so that 5*MAX_PREC does not overflow 32-bit or 64-bit signed 
integers. This eliminates many overflow checks for the exponent.

Updating the exponent is (perhaps surprisingly) quite performance sensitive, 
that's why the 32-bit build does not use a 64-bit exponent.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Stefan Krah]

Stefan Krah  added the comment:

With _pydecimal the memory also grows very slowly (I didn't have the patience 
to wait for MemoryError).

I'm pretty sure decNumber also does the same, it's just easier to implement 
and does not slow down division for small numbers.

libmpdec always favors precisions from roughly 9-34 digits whenever 
there's a potential performance issue.

The only thing I could imagine is to special-case, say, prec > 1.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Vedran Čačić]

Vedran Čačić  added the comment:

Yeah, I should have said "represent" instead of "write". :-)

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Mark Dickinson]

Mark Dickinson  added the comment:

Agreed that this seems surprising.

@Vedran: Trailing zeros in a Decimal object are significant, so 
`Decimal("2.0")` and `Decimal("2.00")` are different (equal, but different). 
The rules about the "ideal exponent" of the result of an arithmetic operation 
are well-specified. In this case, the spec is clear that the answer should be 
`Decimal("2")`, and not `Decimal("2.0")` or `Decimal("2.00")`, or 

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Vedran Čačić]

Vedran Čačić  added the comment:

> Of course the result is exactly 2.  Which I have enough RAM to hold ;-)

You might think so, but if you write it as 2.00...0 with 

>>> decimal.MAX_PREC
99

zeros, I think you're overestimating your RAM capacity. :-P

Now, what is the exact significance of MAX_PREC, I don't know. But I guess some 
hard limits were needed for standards compliance, and Python didn't want to 
limit you there. Of course, same as with recursionlimit, it might be better to 
actually have some reasonable _lower_ limit that we know is actually safe.

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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Batuhan Taskaya]

Change by Batuhan Taskaya :


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[issue39576] Surprising MemoryError in `decimal` with MAX_PREC

[Tim Peters]

New submission from Tim Peters :

Here under Python 3.8.1 on 64-bit Windows:

>>> import decimal
>>> c = decimal.getcontext()
>>> c.prec = decimal.MAX_PREC
>>> i = decimal.Decimal(4)
>>> i / 2
Traceback (most recent call last):
  File "", line 1, in 
MemoryError

Of course the result is exactly 2.  Which I have enough RAM to hold ;-)

The implicit conversion is irrelevant:

>>> i / decimal.Decimal(2)
Traceback (most recent call last):
  File "", line 1, in 
MemoryError

Floor division instead works fine:

>>> i // 2
Decimal('2')

--
components: Library (Lib)
messages: 361536
nosy: tim.peters
priority: normal
severity: normal
status: open
title: Surprising MemoryError in `decimal` with MAX_PREC
versions: Python 3.8, Python 3.9

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