JDK-8229517: Buffered Logging?

Liu, Xin xxinliu at amazon.com
Sat Jan 25 23:37:25 UTC 2020

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.
    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

    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>
    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)

More information about the hotspot-runtime-dev mailing list