JDK-8229517: Buffered Logging?
Thomas Stüfe
thomas.stuefe at gmail.com
Wed Jan 29 11:12:20 UTC 2020
Hi Xin,
I was not sure whether to respond in JBS or here, so I do it here.
Like others I think that if we really do this it would make more sense to
add this as a general feature of UL, not of the GC logging in particular.
We have the problem (logging may block or take time) in other areas too. I
have thought about a similar solution in the past (offloading UL writes to
a dedicated thread) but usually when I thought about the details I got
unsure. But maybe I do not see the whole picture, and you can convince me :)
Some questions (assuming we go the more general direction of buffering UL
output):
- How do you determine which buffer size is large enough? Lets say UL gets
really chatty, how do you deal with buffer overflow? Do you stop the
writers until the buffer is drained or do you discard log output? Both
options seems similar bad.
- We need synchronization now between reader and writers. Slow IO can stall
the reader, which may block the writers (if you stop writers on a full
buffer). I am not sure, but this may be worse than before, where we had the
writer threads effectively write in parallel to the same fd.
Thanks & Cheers, Thomas
On Sun, Jan 26, 2020 at 12:39 AM Liu, Xin <xxinliu at amazon.com> wrote:
> Dear hotspot developers,
>
> I think runtime might be a better forum to continue this discussion.
> The JBS: https://bugs.openjdk.java.net/browse/JDK-8229517
>
> Now our latest patch is here:
> https://cr.openjdk.java.net/~xliu/8229517/webrev02/webrev/
> I know it’s based on jdk8u. I really appreciate if someone reviews it
> and gives me advice. If the community is still interested, we can keep
> developing using better approach or on the tip of jdk.
>
> I’d like to present our experiments here. I try to post data on the
> JDK-8229517, but It doesn't look good in comment area.
>
> Experiments
> We can’t reproduce the problem in [1] because harddisks are not
> accessible for us. To simulate IO blocking in JVM, we introduce a bogus
> delay. It spins DelayInGCLogging milliseconds for every fwrite.
> notproduct(intx, DelayInGCLogging, 0,
> "Delay milliseconds to simulate IO blocking for GC log testing")
>
> It spins in gcLogFileStream::write_blocking.
> https://cr.openjdk.java.net/~xliu/8229517/webrev02/webrev/src/share/vm/utilities/ostream.cpp.cdiff.html
>
> We observe GC pause time using -XX:+PrintGCApplicationStoppedTime for
> JavaWorkload with 2g heap. The unit is second. The program is on github.
> https://github.com/zhenyun/JavaGCworkload.
> $java -Xmx2g -Xms2g -Xloggc:gc.log -XX:+UseG1GC
> -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDetails
> -XX:+UnlockExperimentalVMOptions Test
> sync means jdk8u-dev. async means jdk8u-dev with nonblocking gclog
> change. delayXXX denotes an artifactual delay given by
> -XX:DelayInGCLogging=XXX.
>
> median p95 p99
> p100 losses
> -------------------------------------------------------------------
> async 0.01357 0.36187 0.48691 0.61792
> sync 0.01339 0.3618 0.48646 0.63539
> -------------------------------------------------------------------
> delay5ms-async 0.01516 0.40564 0.486 0.61053 148156
> delay5ms-sync 0.20444 0.4428 0.56733 0.69522
> -------------------------------------------------------------------
> delay10ms-async0.0123 0.47451 0.48623 0.61083 195460
> delay10ms-sync 0.38732 0.52424 0.64783 0.78087
> -------------------------------------------------------------------
>
> async and sync perform equally in happy case. JavaWorkload didn’t lose
> any GC log even though the GC activities are intensive and the outputs are
> verbose. If we introduce 5 ms delay for each file-writing, delay5ms-sync
> starts to show longer stop time for p99 and p100. eg. p99 increases from
> 0.486s to 0.567s, which is 16,7% longer. delay5ms-async remains the same
> p99 and p100 at expense of losing 148k logging events. Because the outputs
> GC logs are contiguous, they are still readable and helpful for problem
> analysis. After we increase delay to 10ms, delay10ms-async still have the
> same p99 and p100 stop time while delay10ms-sync has 22.9% longer stop
> time.
>
> [1]Eliminating Large JVM GC Pauses Caused by Background IO Traffic
>
> https://engineering.linkedin.com/blog/2016/02/eliminating-large-jvm-gc-pauses-caused-by-background-io-traffic
>
> Thanks,
> --lx
>
>
>
>
>
> From: Ramki Ramakrishna <rramakrishna at twitter.com>
> Date: Tuesday, August 13, 2019 at 4:16 PM
> To: "hotspot-dev at openjdk.java.net" <hotspot-dev at openjdk.java.net>
> Cc: "Liu, Xin" <xxinliu at amazon.com>, "Mathiske, Bernd" <
> mathiske at amazon.com>, "Hohensee, Paul" <hohensee at amazon.com>, John Coomes
> <jcoomes at twitter.com>, Tony Printezis <tprintezis at twitter.com>
> Subject: Re: Buffered Logging?
>
>
> I filed https://bugs.openjdk.java.net/browse/JDK-8229517. Please
> correct/complete if I mangled or neglected to fill any specific required
> fields.
>
> Patches forthcoming in the next few days.
> -- ramki
>
> On Tue, Aug 13, 2019 at 2:53 PM Ramki Ramakrishna <mailto:
> rramakrishna at twitter.com> wrote:
>
> I'll go ahead and open a JBS ticket to keep the ball rolling on this
> one, and so we have a place to hang some patches and discussions off of.
> Scream if I shouldn't.
>
> -- ramki
>
> On Mon, Aug 12, 2019 at 6:19 PM Ramki Ramakrishna <mailto:
> rramakrishna at twitter.com> wrote:
>
> (Resent... Apologies to anyone who might see duplicates. There was an
> issue with my membership in the list which, I am guessing, has now been
> resolved.)
>
> ---------- Forwarded message ----------
> From: Ramki Ramakrishna <mailto:rramakrishna at twitter.com>
> To: mailto:hotspot-dev at openjdk.java.net
> Cc: "Liu, Xin" <mailto:xxinliu at amazon.com>, "Mathiske, Bernd" <mailto:
> mathiske at amazon.com>, "Hohensee, Paul" <mailto:hohensee at amazon.com>, John
> Coomes <mailto:jcoomes at twitter.com>, Tony Printezis <mailto:
> tprintezis at twitter.com>
> Bcc:
> Date: Mon, 12 Aug 2019 15:48:17 -0700
> Subject: Buffered Logging?
>
> In the JDK 7 time-frame, some folks noticed much longer than expected
> pauses that seemed to coincide with GC logging in the midst of a GC
> safepoint. In that setup, the GC logs were going to a disk file (these were
> often useful for post-mortem analyses) rather than to a RAM-based tmpfs
> which had been the original design center assumption. The vicissitudes of
> the dirty page flushing policy in Linux when
> IO load on the machine (not necessarily the JVM process doing the
> logging)
> could affect the length and duration of these inline logging stalls.
>
> A buffered logging scheme was then implemented by us (and
> independently by
> others) which we have used successfully to date to avoid these pauses
> in high i/o
> multi-tenant environments.
>
> We have recently ported the OpenJDK 8u based scheme past the Unified
> Logging related changes and have been using it on OpenJDK 11u.
>
> In various fora, some of us have heard people express an interest in
> such an implementation.
>
> Would there be appetite for this in openjdk (there are some
> differences between our scheme in OpenJDK 8u and their subsequent
> post-Unified-Logging port to OpenJDK 11)?
>
> If so, we'd like to open a JBS ticket, attach some patches rebased to
> current OpenJDK tip, and start preparing a webrev for discussion with a
> view to possible upstreaming if it converges past review/discussion/debate.
>
> PS: In recent discussion at the JVMLS, mention was made of something
> that might be similar in support of JFR streaming, so our rebase to tip and
> familiarization with any JFR-associated buffered logging might well make
> this easier or, possibly, moot.
>
> Thoughts, questions, comments, feedback?
> -- ramki
>
> --
> JVM Team, Platform Engineering, Twitter (San Francisco)
>
>
>
> --
> JVM Team, Infrastructure Engineering, Twitter (San Francisco)
>
>
>
> --
> JVM Team, Infrastructure Engineering, Twitter (San Francisco)
>
>
>
>
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