Incremental:
http://cr.openjdk.java.net/~rrich/webrevs/2019/8227745/webrev.4.inc/
I was not able to eliminate the additional suspend flag now. I'll take care of
this
as soon as the
existing suspend-resume-mechanism is reworked.
Testing:
Nightly tests @SAP:
JCK and JTREG, also in Xcomp mode, SPECjvm2008, SPECjbb2015, Renaissance
Suite, SAP specific tests
with fastdebug and release builds on all platforms
Stress testing with DeoptimizeObjectsALot running SPECjvm2008 40x parallel
for 24h
Thanks, Richard.
More details on the changes:
* Hide DeoptimizeObjectsALotThread from external view.
* Changed EscapeBarrier_lock to be a _safepoint_check_never lock.
It used to be _safepoint_check_sometimes, which will be eliminated sooner or
later.
I added explicit thread state changes with ThreadBlockInVM to code paths
where we can wait()
on EscapeBarrier_lock to become safepoint safe.
* Use handshake EscapeBarrierSuspendHandshake to suspend target threads
instead of vm operation
VM_ThreadSuspendAllForObjDeopt.
* Removed uses of Threads_lock. When adding a new thread we suspend it iff
EA optimizations are
being reverted. In the previous version we were waiting on Threads_lock
while EA optimizations
were reverted. See EscapeBarrier::thread_added().
* Made tests require Xmixed compilation mode.
* Made tests agnostic regarding tiered compilation.
I.e. tc isn't disabled anymore, and the tests can be run with tc enabled or
disabled.
* Exercising EATests.java as well with stress test options
DeoptimizeObjectsALot*
Due to the non-deterministic deoptimizations some tests need to be skipped.
We do this to prevent bit-rot of the stress test code.
* Executing EATests.java as well with graal if available. Driver for this is
EATestsJVMCI.java. Graal cannot pass all tests, because it does not provide
all
the new debug info
(namely not_global_escape_in_scope and arg_escape in scopeDesc.hpp).
And graal does not yet support the JVMTI operations force early return and
pop frame.
* Removed tracing from new jdi tests in EATests.java. Too much trace output
before the debugging
connection is established can cause deadlock because output buffers fill up.
(See https://bugs.openjdk.java.net/browse/JDK-8173304)
* Many copyright year changes and smaller clean-up changes of testing code
(trailing white-space and
the like).
-----Original Message-----
From: David Holmes <david.hol...@oracle.com>
Sent: Donnerstag, 19. Dezember 2019 03:12
To: Reingruber, Richard <richard.reingru...@sap.com>; serviceability-
d...@openjdk.java.net; hotspot-compiler-...@openjdk.java.net; hotspot-
runtime-...@openjdk.java.net; Vladimir Kozlov (vladimir.koz...@oracle.com)
<vladimir.koz...@oracle.com>
Subject: Re: RFR(L) 8227745: Enable Escape Analysis for Better Performance in
the Presence of JVMTI Agents
Hi Richard,
I think my issue is with the way EliminateNestedLocks works so I'm going
to look into that more deeply.
Thanks for the explanations.
David
On 18/12/2019 12:47 am, Reingruber, Richard wrote:
Hi David,
> > > Some further queries/concerns:
> > >
> > > src/hotspot/share/runtime/objectMonitor.cpp
> > >
> > > Can you please explain the changes to ObjectMonitor::wait:
> > >
> > > ! _recursions = save // restore the old recursion count
> > > ! + jt->get_and_reset_relock_count_after_wait(); //
> > > increased by the deferred relock count
> > >
> > > what is the "deferred relock count"? I gather it relates to
> > >
> > > "The code was extended to be able to deoptimize objects of a
> > frame that
> > > is not the top frame and to let another thread than the owning
> > thread do
> > > it."
> >
> > Yes, these relate. Currently EA based optimizations are reverted, when a
compiled frame is
> > replaced with corresponding interpreter frames. Part of this is
relocking
objects with eliminated
> > locking. New with the enhancement is that we do this also just before
object references are
> > acquired through JVMTI. In this case we deoptimize also the owning
compiled frame C and we
> > register deoptimized objects as deferred updates. When control returns
to C it gets deoptimized,
> > we notice that objects are already deoptimized (reallocated and
relocked), so we don't do it again
> > (relocking twice would be incorrect of course). Deferred updates are
copied into the new
> > interpreter frames.
> >
> > Problem: relocking is not possible if the target thread T is waiting on
the
monitor that needs to
> > be relocked. This happens only with non-local objects with
EliminateNestedLocks. Instead relocking
> > is deferred until T owns the monitor again. This is what the piece of
code above does.
>
> Sorry I need some more detail here. How can you wait() on an object
> monitor if the object allocation and/or locking was optimised away? And
> what is a "non-local object" in this context? Isn't EA restricted to
> thread-confined objects?
"Non-local object" is an object that escapes its thread. The issue I'm
addressing with the changes
in ObjectMonitor::wait are almost unrelated to EA. They are caused by
EliminateNestedLocks, where C2
eliminates recursive locking of an already owned lock. The lock owning object
exists on the heap, it
is locked and you can call wait() on it.
EliminateLocks is the C2 option that controls lock elimination based on EA.
Both optimizations have
in common that objects with eliminated locking need to be relocked when
deoptimizing a frame,
i.e. when replacing a compiled frame with equivalent interpreter
frames. Deoptimization::relock_objects does that job for /all/ eliminated
locks in scope. /All/ can
be a mix of eliminated nested locks and locks of not-escaping objects.
New with the enhancement: I call relock_objects earlier, just before objects
pontentially
escape. But then later when the owning compiled frame gets deoptimized, I
must not do it again:
See call to EscapeBarrier::objs_are_deoptimized in deoptimization.cpp:
373 if ((jvmci_enabled || ((DoEscapeAnalysis || EliminateNestedLocks) &&
EliminateLocks))
374 && !EscapeBarrier::objs_are_deoptimized(thread, deoptee.id())) {
375 bool unused;
376 eliminate_locks(thread, chunk, realloc_failures, deoptee, exec_mode,
unused);
377 }
Now when calling relock_objects early it is quiet possible that I have to relock
an object the
target thread currently waits for. Obviously I cannot relock in this case,
instead I chose to
introduce relock_count_after_wait to JavaThread.
> Is it just that some of the locking gets optimized away e.g.
>
> synchronised(obj) {
> synchronised(obj) {
> synchronised(obj) {
> obj.wait();
> }
> }
> }
>
> If this is reduced to a form as-if it were a single lock of the monitor
> (due to EA) and the wait() triggers a JVM TI event which leads to the
> escape of "obj" then we need to reconstruct the true lock state, and so
> when the wait() internally unblocks and reacquires the monitor it has to
> set the true recursion count to 3, not the 1 that it appeared to be when
> wait() was initially called. Is that the scenario?
Kind of... except that the locking is not eliminated due to EA and there is no
JVM TI event
triggered by wait.
Add
LocalObject l1 = new LocalObject();
in front of the synchrnized blocks and assume a JVM TI agent acquires l1. This
triggers the code in
question.
See that relocking/reallocating is transactional. If it is done then for /all/
objects in scope and it is
done at most once. It wouldn't be quite so easy to split this in relocking of
nested/EA-based
eliminated locks.
> If so I find this truly awful. Anyone using wait() in a realistic form
> requires a notification and so the object cannot be thread confined. In
It is not thread confined.
> which case I would strongly argue that upon hitting the wait() the deopt
> should occur unconditionally and so the lock state is correct before we
> wait and so we don't need to mess with the recursion count internally
> when we reacquire the monitor.
>
> >
> > > which I don't like the sound of at all when it comes to
ObjectMonitor
> > > state. So I'd like to understand in detail exactly what is going
on here
> > > and why. This is a very intrusive change that seems to badly break
> > > encapsulation and impacts future changes to ObjectMonitor that are
under
> > > investigation.
> >
> > I would not regard this as breaking encapsulation. Certainly not badly.
> >
> > I've added a property relock_count_after_wait to JavaThread. The
property is well
> > encapsulated. Future ObjectMonitor implementations have to deal with
recursion too. They are free
> > in choosing a way to do that as long as that property is taken into
account. This is hardly a
> > limitation.
>
> I do think this badly breaks encapsulation as you have to add a callout
> from the guts of the ObjectMonitor code to reach into the thread to get
> this lock count adjustment. I understand why you have had to do this but
> I would much rather see a change to the EA optimisation strategy so that
> this is not needed.
>
> > Note also that the property is a straight forward extension of the
existing concept of deferred
> > local updates. It is embedded into the structure holding them. So not
even the footprint of a
> > JavaThread is enlarged if no deferred updates are generated.
>
> [...]
>
> >
> > I'm actually duplicating the existing external suspend mechanism,
because a thread can be
> > suspended at most once. And hey, and don't like that either! But it
seems not unlikely that the
> > duplicate can be removed together with the original and the new type
of handshakes that will be
> > used for thread suspend can be used for object deoptimization too. See
today's discussion in
> > JDK-8227745 [2].
>
> I hope that discussion bears some fruit, at the moment it seems not to
> be possible to use handshakes here. :(
>
> The external suspend mechanism is a royal pain in the proverbial that we
> have to carefully live with. The idea that we're duplicating that for
> use in another fringe area of functionality does not thrill me at all.
>
> To be clear, I understand the problem that exists and that you wish to
> solve, but for the runtime parts I balk at the complexity cost of
> solving it.
I know it's complex, but by far no rocket science.
Also I find it hard to imagine another fix for JDK-8233915 besides changing
the JVM TI specification.
Thanks, Richard.
-----Original Message-----
From: David Holmes <david.hol...@oracle.com>
Sent: Dienstag, 17. Dezember 2019 08:03
To: Reingruber, Richard <richard.reingru...@sap.com>; serviceability-
d...@openjdk.java.net; hotspot-compiler-...@openjdk.java.net; hotspot-
runtime-...@openjdk.java.net; Vladimir Kozlov (vladimir.koz...@oracle.com)
<vladimir.koz...@oracle.com>
Subject: Re: RFR(L) 8227745: Enable Escape Analysis for Better Performance
in the Presence of JVMTI Agents
<resend as my mailer crashed during last send>
David
On 17/12/2019 4:57 pm, David Holmes wrote:
Hi Richard,
On 14/12/2019 5:01 am, Reingruber, Richard wrote:
Hi David,
> Some further queries/concerns:
>
> src/hotspot/share/runtime/objectMonitor.cpp
>
> Can you please explain the changes to ObjectMonitor::wait:
>
> ! _recursions = save // restore the old recursion count
> ! + jt->get_and_reset_relock_count_after_wait(); //
> increased by the deferred relock count
>
> what is the "deferred relock count"? I gather it relates to
>
> "The code was extended to be able to deoptimize objects of a
frame that
> is not the top frame and to let another thread than the owning
thread do
> it."
Yes, these relate. Currently EA based optimizations are reverted, when
a compiled frame is replaced
with corresponding interpreter frames. Part of this is relocking
objects with eliminated
locking. New with the enhancement is that we do this also just before
object references are acquired
through JVMTI. In this case we deoptimize also the owning compiled
frame C and we register
deoptimized objects as deferred updates. When control returns to C it
gets deoptimized, we notice
that objects are already deoptimized (reallocated and relocked), so we
don't do it again (relocking
twice would be incorrect of course). Deferred updates are copied into
the new interpreter frames.
Problem: relocking is not possible if the target thread T is waiting
on the monitor that needs to be
relocked. This happens only with non-local objects with
EliminateNestedLocks. Instead relocking is
deferred until T owns the monitor again. This is what the piece of
code above does.
Sorry I need some more detail here. How can you wait() on an object
monitor if the object allocation and/or locking was optimised away? And
what is a "non-local object" in this context? Isn't EA restricted to
thread-confined objects?
Is it just that some of the locking gets optimized away e.g.
synchronised(obj) {
synchronised(obj) {
synchronised(obj) {
obj.wait();
}
}
}
If this is reduced to a form as-if it were a single lock of the monitor
(due to EA) and the wait() triggers a JVM TI event which leads to the
escape of "obj" then we need to reconstruct the true lock state, and so
when the wait() internally unblocks and reacquires the monitor it has to
set the true recursion count to 3, not the 1 that it appeared to be when
wait() was initially called. Is that the scenario?
If so I find this truly awful. Anyone using wait() in a realistic form
requires a notification and so the object cannot be thread confined. In
which case I would strongly argue that upon hitting the wait() the deopt
should occur unconditionally and so the lock state is correct before we
wait and so we don't need to mess with the recursion count internally
when we reacquire the monitor.
> which I don't like the sound of at all when it comes to
ObjectMonitor
> state. So I'd like to understand in detail exactly what is going
on here
> and why. This is a very intrusive change that seems to badly break
> encapsulation and impacts future changes to ObjectMonitor that
are under
> investigation.
I would not regard this as breaking encapsulation. Certainly not badly.
I've added a property relock_count_after_wait to JavaThread. The
property is well
encapsulated. Future ObjectMonitor implementations have to deal with
recursion too. They are free in
choosing a way to do that as long as that property is taken into
account. This is hardly a
limitation.
I do think this badly breaks encapsulation as you have to add a callout
from the guts of the ObjectMonitor code to reach into the thread to get
this lock count adjustment. I understand why you have had to do this but
I would much rather see a change to the EA optimisation strategy so that
this is not needed.
Note also that the property is a straight forward extension of the
existing concept of deferred
local updates. It is embedded into the structure holding them. So not
even the footprint of a
JavaThread is enlarged if no deferred updates are generated.
> ---
>
> src/hotspot/share/runtime/thread.cpp
>
> Can you please explain why
JavaThread::wait_for_object_deoptimization
> has to be handcrafted in this way rather than using proper
transitions.
>
I wrote wait_for_object_deoptimization taking
JavaThread::java_suspend_self_with_safepoint_check
as template. So in short: for the same reasons :)
Threads reach both methods as part of thread state transitions,
therefore special handling is
required to change thread state on top of ongoing transitions.
> We got rid of "deopt suspend" some time ago and it is disturbing
to see
> it being added back (effectively). This seems like it may be
something
> that handshakes could be used for.
Deopt suspend used to be something rather different with a similar
name[1]. It is not being added back.
I stand corrected. Despite comments in the code to the contrary
deopt_suspend didn't actually cause a self-suspend. I was doing a lot of
cleanup in this area 13 years ago :)
I'm actually duplicating the existing external suspend mechanism,
because a thread can be suspended
at most once. And hey, and don't like that either! But it seems not
unlikely that the duplicate can
be removed together with the original and the new type of handshakes
that will be used for
thread suspend can be used for object deoptimization too. See today's
discussion in JDK-8227745 [2].
I hope that discussion bears some fruit, at the moment it seems not to
be possible to use handshakes here. :(
The external suspend mechanism is a royal pain in the proverbial that we
have to carefully live with. The idea that we're duplicating that for
use in another fringe area of functionality does not thrill me at all.
To be clear, I understand the problem that exists and that you wish to
solve, but for the runtime parts I balk at the complexity cost of
solving it.
Thanks,
David
-----
Thanks, Richard.
[1] Deopt suspend was something like an async. handshake for
architectures with register windows,
where patching the return pc for deoptimization of a compiled
frame was racy if the owner thread
was in native code. Instead a "deopt" suspend flag was set on
which the thread patched its own
frame upon return from native. So no thread was suspended. It got
its name only from the name of
the flags.
[2] Discussion about using handshakes to sync. with the target thread:
https://bugs.openjdk.java.net/browse/JDK-
8227745?focusedCommentId=14306727&page=com.atlassian.jira.plugin.syste
m.issuetabpanels:comment-tabpanel#comment-14306727
-----Original Message-----
From: David Holmes <david.hol...@oracle.com>
Sent: Freitag, 13. Dezember 2019 00:56
To: Reingruber, Richard <richard.reingru...@sap.com>;
serviceability-dev@openjdk.java.net;
hotspot-compiler-...@openjdk.java.net;
hotspot-runtime-...@openjdk.java.net
Subject: Re: RFR(L) 8227745: Enable Escape Analysis for Better
Performance in the Presence of JVMTI Agents
Hi Richard,
Some further queries/concerns:
src/hotspot/share/runtime/objectMonitor.cpp
Can you please explain the changes to ObjectMonitor::wait:
! _recursions = save // restore the old recursion count
! + jt->get_and_reset_relock_count_after_wait(); //
increased by the deferred relock count
what is the "deferred relock count"? I gather it relates to
"The code was extended to be able to deoptimize objects of a frame that
is not the top frame and to let another thread than the owning thread do
it."
which I don't like the sound of at all when it comes to ObjectMonitor
state. So I'd like to understand in detail exactly what is going on here
and why. This is a very intrusive change that seems to badly break
encapsulation and impacts future changes to ObjectMonitor that are under
investigation.
---
src/hotspot/share/runtime/thread.cpp
Can you please explain why JavaThread::wait_for_object_deoptimization
has to be handcrafted in this way rather than using proper transitions.
We got rid of "deopt suspend" some time ago and it is disturbing to see
it being added back (effectively). This seems like it may be something
that handshakes could be used for.
Thanks,
David
-----
On 12/12/2019 7:02 am, David Holmes wrote:
On 12/12/2019 1:07 am, Reingruber, Richard wrote:
Hi David,
> Most of the details here are in areas I can comment on in detail,
but I
> did take an initial general look at things.
Thanks for taking the time!
Apologies the above should read:
"Most of the details here are in areas I *can't* comment on in detail
..."
David
> The only thing that jumped out at me is that I think the
> DeoptimizeObjectsALotThread should be a hidden thread.
>
> + bool is_hidden_from_external_view() const { return true; }
Yes, it should. Will add the method like above.
> Also I don't see any testing of the DeoptimizeObjectsALotThread.
Without
> active testing this will just bit-rot.
DeoptimizeObjectsALot is meant for stress testing with a larger
workload. I will add a minimal test
to keep it fresh.
> Also on the tests I don't understand your @requires clause:
>
> @requires ((vm.compMode != "Xcomp") & vm.compiler2.enabled
&
> (vm.opt.TieredCompilation != true))
>
> This seems to require that TieredCompilation is disabled, but
tiered is
> our normal mode of operation. ??
>
I removed the clause. I guess I wanted to target the tests towards the
code they are supposed to
test, and it's easier to analyze failures w/o tiered compilation and
with just one compiler thread.
Additionally I will make use of
compiler.whitebox.CompilerWhiteBoxTest.THRESHOLD in the tests.
Thanks,
Richard.
-----Original Message-----
From: David Holmes <david.hol...@oracle.com>
Sent: Mittwoch, 11. Dezember 2019 08:03
To: Reingruber, Richard <richard.reingru...@sap.com>;
serviceability-dev@openjdk.java.net;
hotspot-compiler-...@openjdk.java.net;
hotspot-runtime-...@openjdk.java.net
Subject: Re: RFR(L) 8227745: Enable Escape Analysis for Better
Performance in the Presence of JVMTI Agents
Hi Richard,
On 11/12/2019 7:45 am, Reingruber, Richard wrote:
Hi,
I would like to get reviews please for
http://cr.openjdk.java.net/~rrich/webrevs/2019/8227745/webrev.3/
Corresponding RFE:
https://bugs.openjdk.java.net/browse/JDK-8227745
Fixes also https://bugs.openjdk.java.net/browse/JDK-8233915
And potentially https://bugs.openjdk.java.net/browse/JDK-8214584 [1]
Vladimir Kozlov kindly put webrev.3 through tier1-8 testing without
issues (thanks!). In addition the
change is being tested at SAP since I posted the first RFR some
months ago.
The intention of this enhancement is to benefit performance wise from
escape analysis even if JVMTI
agents request capabilities that allow them to access local variable
values. E.g. if you start-up
with -agentlib:jdwp=transport=dt_socket,server=y,suspend=n, then
escape analysis is disabled right
from the beginning, well before a debugger attaches -- if ever one
should do so. With the
enhancement, escape analysis will remain enabled until and after a
debugger attaches. EA based
optimizations are reverted just before an agent acquires the
reference to an object. In the JBS item
you'll find more details.
Most of the details here are in areas I can comment on in detail, but I
did take an initial general look at things.
The only thing that jumped out at me is that I think the
DeoptimizeObjectsALotThread should be a hidden thread.
+ bool is_hidden_from_external_view() const { return true; }
Also I don't see any testing of the DeoptimizeObjectsALotThread.
Without
active testing this will just bit-rot.
Also on the tests I don't understand your @requires clause:
@requires ((vm.compMode != "Xcomp") & vm.compiler2.enabled &
(vm.opt.TieredCompilation != true))
This seems to require that TieredCompilation is disabled, but tiered is
our normal mode of operation. ??
Thanks,
David
Thanks,
Richard.
[1] Experimental fix for JDK-8214584 based on JDK-8227745
http://cr.openjdk.java.net/~rrich/webrevs/2019/8214584/experiment_v1.patc
h
