<html><head><style>body{font-family:Helvetica,Arial;font-size:13px}</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Jon,</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">OK, I’m working on porting the CMS changes for 1) to JDK 9. I’ll have a web rev I can share soonish.</div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;"><br></div><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Tony</div> <br><p class="airmail_on">On January 11, 2016 at 4:31:03 PM, Jon Masamitsu (<a href="mailto:jon.masamitsu@oracle.com">jon.masamitsu@oracle.com</a>) wrote:</p> <blockquote type="cite" class="clean_bq"><span><div text="#000000" bgcolor="#FFFFFF"><div></div><div>
<title></title>
<font face="Times New Roman, Times, serif">Tony,<br>
<br>
We'd be interested in the fix for 1). I'll have to go
look at more code<br>
before having a definite opinion on 2) but the way you
describe it<br>
makes it sound like something worth doing. Similarly
with 3).<br>
<br>
Jon<br>
<br></font><br>
<div class="moz-cite-prefix">On 01/11/2016 09:59 AM, Tony Printezis
wrote:<br></div>
<blockquote cite="mid:etPan.5693ed80.4a73d629.58e@tw-mbp-tprintezis" type="cite">
<div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">
<div id="bloop_customfont" style="margin: 0px;">Hi all,</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">We have been
recently investigating some very lengthy (several minutes)
promotion failures in ParNew, which also appear in ParallelGC. We
have identified a few issues and have some fixes to address them.
Here's a quick summary:</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">1) There's a
scalability bottleneck when adding marks to the preserved mark
stack as there is only one stack, shared by all workers, and pushes
to it are protected by a mutex. This essentially serializes all
workers if there is a non-trivial amount of marks to be preserved.
The fix is similar to what's been implemented in G1 in JDK 9, which
is to introduce per-worker preserved mark stacks.</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">2) (More
interestingly) I was perplexed by the huge number of marks that I
see getting preserved during promotion failure. I did a small study
with a test I can reproduce the issue with. The majority of the
preserved marks were 0x5 (i.e. "anonymously biased"). According to
the current logic, no mark is preserved if it's biased, presumably
because it's assumed that the object is biased towards a specific
thread and we want to preserve that mark as it contains the thread
pointer. The fix is to use a different default mark value when
biased locking is enabled (0x5) or disabled (0x1, as it is now).
During promotion failures, marks are not preserved if they are
equal to the default value and the mark of forwarded objects is set
to the default value post promotion failure and before the
preserved marks are re-instated.</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">A few extra
observations on this:</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">- I don't know if
the majority of objects we'll come across during promotion failures
will be anonymously biased (it is the case for synthetic
benchmarks). So, the above might pay off in certain cases but not
all. But I think it's still worth doing.</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">- Even though the
per-worker preserved mark stacks eliminate the big scalability
bottleneck, reducing (potentially dramatically) the number of marks
that are preserved helps in a couple of ways: a) avoids allocating
a lot of memory for the preserved mark stacks (which can get very,
very large in some cases) and b) avoids having to scan / reclaim
the preserved mark stacks post promotion failure, which reduces the
overall GC time further. Even the parallel time in ParNew improves
by a bit because there are a lot fewer stack pushes and malloc
calls.</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">3) In the case
where lots of marks need to be preserved, we found that using 64K
stack segments, instead of 4K segments, speeds up the preserved
mark stack reclamation by a non-trivial amount (it's 3x/4x
faster).</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">We have fixes for
all three issues above for ParNew. We're also going to implement
them for ParallelGC. For JDK 9, 1) is already implemented, but 2)
or 3) might also be worth doing.</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">Is there interest
in these changes?</div>
<div id="bloop_customfont" style="margin: 0px;"><br></div>
<div id="bloop_customfont" style="margin: 0px;">Tony</div>
<div><br></div>
</div>
<br>
<div id="bloop_sign_1452534850323611136" class="bloop_sign">
<div style="font-family:helvetica,arial;font-size:13px">
<div>-----</div>
<div><br></div>
<div>Tony Printezis | JVM/GC Engineer / VM Team | Twitter</div>
<div><br></div>
<div>@TonyPrintezis</div>
<div><a moz-do-not-send="true" href="mailto:tprintezis@twitter.com">tprintezis@twitter.com</a></div>
<div><br></div>
</div>
</div>
</blockquote>
<br>
</div></div></span></blockquote> <div id="bloop_sign_1452621353147041024" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px"><div>-----</div><div><br></div><div>Tony Printezis | JVM/GC Engineer / VM Team | Twitter</div><div><br></div><div>@TonyPrintezis</div><div><a href="mailto:tprintezis@twitter.com">tprintezis@twitter.com</a></div><div><br></div></div></div></body></html>