Hi John --
It's hard to say without further information, particularly about the
ratio of remote references vs. local ones, but a 15-20x slowdown
wouldn't be unusual in our experience. What is your timing technique?
Are you using the built-in Chapel timing support, or are you timing the
entire execution from outside the program, or? (You may have mentioned
this earlier and I didn't catch it.)
As to specifics ... you can probably skip collecting the init comm
stats, since most of that is built-in module initialization that is
outside your control. In the runn comm diags, 1600 of the 1604 remote
forks done by locale 1 are presumably its half of 3200 writeln() calls
that print the results. The number of get_nb (nonblocking GET)
operations seems high for a program that is supposed to only to 1000
remote loads on each locale. I take it this was run with the remote
cache enabled? If it were disabled I would expect all of the get_nb
values to be reported as plain gets instead, since a side effect of
enabling the remote cache is to turn blocking remote refs into
nonblocking ones. You might try running with the remote cache disabled
to see how performance and comm diags results compare.
greg
On 2/19/2015 12:06 PM, John MacFrenz wrote:
Hi,
So... I did fiddle with chapel settings, booted machines and as
another node was running under virtualbox I tried different settings
for it. Somehow the problem got fixed, and I'm unable to reproduce it
any more though I set exact same settings I had before. Well, if I
face this issue again I try to gather more information about it...
Might also be that booting helped, though I had already tried that
once. Now the execution is still about 15-20 times slower, though I've
got an impression that this is what could be excepted from chapel?
Nodes had 4 and 2 cores (with HT 8 and 4) and 6GB and 4GB memory.
Memory usage of the program was very small, 3 megabytes per node. In
that setup I tried one locale per node, but I also tried using single
system with 2 to 4 locales.
I also got comm diagnostics, though I'm afraid I don't have enough
knowledge to inteprete results. here's the results for two locales
Init comm diagnostics
(get = 0, get_nb = 0, put = 0, put_nb = 0, test_nb = 0, wait_nb = 0,
try_nb = 0, fork = 12, fork_fast = 0, fork_nb = 13) (get = 0, get_nb =
175, put = 0, put_nb = 8, test_nb = 0, wait_nb = 0, try_nb = 0, fork =
17, fork_fast = 0, fork_nb = 0)
Run comm diagnostics
(get = 0, get_nb = 4222, put = 0, put_nb = 0, test_nb = 0, wait_nb =
0, try_nb = 0, fork = 0, fork_fast = 0, fork_nb = 1602) (get = 0,
get_nb = 7218, put = 0, put_nb = 800, test_nb = 0, wait_nb = 0, try_nb
= 0, fork = 1604, fork_fast = 0, fork_nb = 0)
During the run (excluding initialization), each locale should get
about 1000 real(64) values from another locale. On top of that results
are printed with writeln with GASNET_ROUTE_OUTPUT=0, with total of
about 3200 reals printed per locale which is included in run comm
diagnostics. Do those comm statistics seem reasonable for that?
19.02.2015, 18:58, "Michael Ferguson" <[email protected]
<mailto:[email protected]>>:
Hi John -
How many cores does your system have? How much memory? How much
memory is the program using? How many locales are you launching on
the single system? How many threads are you assigning to each locale?
I’d bet that your problem is either from different threads contending
over the same processor resources (which you can limit as Greg
pointed out with CHPL_RT_NUM_THREADS_PER_LOCALE ) or using to much
memory since you are running many locales on a single machine – and
Chapel doesn’t currently try to reduce its per-locale resources when
oversubscribed in this manner.
Of course, it could be the communication as well – you can check
that. You can also instrument your program to print out communication
counts (as I described in an earlier email – try mirroring the use of
CommDiagnostics in
chapel-lang-github/test/performance/ferguson/remote-class-read.chpl )
You can also try running on a real cluster…
Cheers,
-michael
From: Greg Titus <[email protected] <mailto:[email protected]>>
Date: Thursday, February 19, 2015 at 11:41 AM
To: John MacFrenz <[email protected] <mailto:[email protected]>>
Cc: Michael Ferguson <[email protected]
<mailto:[email protected]>>,
"[email protected]
<mailto:[email protected]>"
<[email protected]
<mailto:[email protected]>>
Subject: Re: Variable Block Distributions
Folks here pointed out a mistake I made in the discussion below: the
environment variable that sets the number of threads and thus
processor cores to use is CHPL_RT_NUM_THREADS_PER_LOCALE. I left off
the '_RT' part of that variable name below. It won't work if it's not
spelled right! :-)
greg
On 2/19/2015 8:45 AM, Greg Titus wrote:
Hi John --
(You already know the following as general knowledge, but I thought
I'd include it for people newer to multi-locale programming who may
be following this conversation.)
For context, with current Chapel it's normal for programs to suffer
some performance loss when moving from single-locale to multi-locale
execution. Using multiple locales offers more opportunity for
parallelism, but at the cost of reduced intra-task performance due
to network communication required for inter-locale variable
references. The effect varies across programs depending on how much
remote communication they do, but since a remote reference can
easily take 1000 times as long as a local one, it doesn't take much
to have a big effect. For example, with Chapel 1.10 on a Cray XC we
don't see any drop-off in performance from 1 locale to 2 on the
Stream benchmark, but our Stream doesn't do many inter-locale memory
references. But for the RA benchmark, which does a lot of
inter-locale references (in fact that's what it's measuring), our
multi-locale performance doesn't match that on a single locale until
we get up to 8-32 locales, depending on circumstances. And the Cray
XC has a very high-performance network compared to UDP or MPI over
ethernet.
That said, the >100x slowdown you're seeing seems a little high,
unless your test case is really doing a lot of remote references. If
it isn't, or at least shouldn't be, perhaps you're seeing a lot of
remote communication for internal references to meta-data, within
your distribution code? If this is the case, then turning on remote
caching could well improve matters. In fact that might be a good
test to rule this hypothesis in or out.
A secondary effect with GASNET_SPAWNFN=L could be oversubscription
of the processor cores due to running more than one Chapel locale
per compute node. To reduce the level of oversubscription you could
set CHPL_NUM_THREADS_PER_LOCALE to the number of compute-node cores
divided by the number of locales you're running on the compute node,
but don't set it to less than 2 or you could deadlock/livelock due
to internal starvation. However, if you're seeing the same slowdown
with GASNET_SPAWNFN=S and one Chapel locale per compute node then I
don't think this is something that is afflicting you right now.
hope this helps,
greg
On 2/19/2015 4:42 AM, John MacFrenz wrote:
Hi,
I'll give --cache-remote a try later. However for now I'm facing
some problems which definitely should be solved first...
The problem I'm having is that using GASNET_SPAWNF=L with
UDP-conduit with more than one locale causes program to run _very_
slowly. For example, with one locale my test program did take 0.20
sec to run. With two locales it took 65 seconds. Same can be
observed when running with GASNET_SPAWNF=S with UDP conduit on two
separate machines. Using MPI conduit didn't make difference. Here's
the environment variables I'm using
CHPL_HOME: /home/share/chapel/chapel-git
script location: /home/share/chapel/chapel-git/util
CHPL_HOST_PLATFORM: linux32
CHPL_HOST_COMPILER: gnu
CHPL_TARGET_PLATFORM: linux32
CHPL_TARGET_COMPILER: gnu
CHPL_TARGET_ARCH: unknown
CHPL_LOCALE_MODEL: flat
CHPL_COMM: gasnet
CHPL_COMM_SUBSTRATE: udp
CHPL_GASNET_SEGMENT: everything
CHPL_TASKS: fifo
CHPL_LAUNCHER: amudprun
CHPL_TIMERS: generic
CHPL_MEM: cstdlib
CHPL_MAKE: gmake
CHPL_ATOMICS: intrinsics
CHPL_NETWORK_ATOMICS: none
CHPL_GMP: none
CHPL_HWLOC: none
CHPL_REGEXP: none
CHPL_WIDE_POINTERS: struct
CHPL_LLVM: none
CHPL_AUX_FILESYS: none
Any idea what could be causing this? As I said on some previous
post my target environment is heterogeneous (all x86, though)
commodity cluster with ethernet connections, so either UDP or MPI
conduit is one I'd use..
18.02.2015, 23:41, "Greg Titus" <[email protected]> <mailto:[email protected]>:
Hi John --
A little bit of follow-up to what Michael says here ...
The "nemesis" he refers to is the internal name of the particular
Qthreads scheduler we use when CHPL_LOCALE_MODEL=flat. Our
understanding is that the nemesis scheduler currently doesn't move
qthreads (and by extension, Chapel tasks) from pthread to pthread,
which would break the use of pthread local storage inside the
remote caching implementation. But there are significant caveats here:
* We use a different Qthreads scheduler when
CHPL_LOCALE_MODEL=numa, and that one definitely does move
qthreads (thus Chapel tasks) from pthread to pthread.
* We can't guarantee that we'll always use "nemesis" with the
flat locale model.
* We can't guarantee that, even if we do keep using it,
"nemesis" will continue to not move qthreads (thus Chapel
tasks) from pthread to pthread.
Taken together, this basically says that although we haven't
observed remote caching failures with qthreads, that shouldn't be
taken as evidence that it definitely does work now or will work in
the future.
greg
On 2/18/2015 2:31 PM, Michael Ferguson wrote:
Hi -
One more thing about the --cache-remote feature, just to be clear
and for future reference:
The remote caching depends on pthread local storage, and Chapel
task movement among worker pthreads in Qthreads-based tasking
could break it. So far we haven't seen this happen, but we cannot
guarantee it won't. Symptoms of a failure could include silent
wrong answers or segfaults, either of which could be solid or
intermittent/sporadic.
I *think* that this problem won't come up with the nemesis
qthreads scheduler, but we need to do some careful analysis
before we can declare the --cache-remote feature safe to use with
qthreads.
Cheers,
-michael
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