<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">The mutator processing doesn't solve it. The card clearing event is still asynchronous with respect to possible mutations in other threads. While one mutator thread is processing buffers and clearing cards the other can sneak in and do the store to the same object that will go unnoticed. So I'm afraid it's either a store-load barrier, or we need to stop all mutator threads to prevent this race, or worse..<div><br></div><div>igor<br><div><br></div><div><br><div><div>On Jun 28, 2013, at 1:53 PM, John Cuthbertson <<a href="mailto:john.cuthbertson@oracle.com">john.cuthbertson@oracle.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
  
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    Hi Igor,<br>
    <br>
    Yeah G1 has that facility right now. In fact you added it. :) When
    the number of completed buffers is below the green zone upper limit,
    none of the refinement threads are refining buffers. That is the
    green zone upper limit is number of buffers that we expect to be
    able to process during the GC without it going over some percentage
    of the pause time (I think the default is 10%). When the number of
    buffers grows above the green zone upper limit, the refinement
    threads start processing the buffers in stepped manner. <br>
    <br>
    So during the safepoint we would process N - green-zone-upper-limit
    completed buffers. In fact we could have a watcher task that
    monitors the number of completed buffers and triggers a safepoint
    when the number of completed buffers becomes sufficiently high - say
    above the yellow-zone upper limit.<br>
    <br>
    That does away with the whole notion of concurrent refinement but
    will remove a lot of the nasty complicated code that gets executed
    by the mutators or refinement threads.<br>
    <br>
    My main concern is that the we would be potentially  increasing the
    number and duration of non-GC safepoints which cause issues with
    latency sensitive apps. For those workloads that only care about 90%
    of the transactions this approach would probably be fine.<br>
    <br>
    We would need to evaluate the performance of each approach. <br>
    <br>
    The card cache delays the processing of cards that have been dirtied
    multiple times - so it does act kind of like a buffer reducing the
    potential for this issue.<br>
    <br>
    JohnC<br>
    <br>
    <div class="moz-cite-prefix">On 6/28/2013 12:47 PM, Igor Veresov
      wrote:<br>
    </div>
    <blockquote cite="mid:AA908111-B24A-45E5-B536-F9D5356F77B7@gmail.com" type="cite">
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      The impact on the next collection however can be bounded. Say, if
      you make it have a safepoint to reap the buffers when the number
      of buffer reaches $n$, that alone would put a cap on the potential
      pause incurred during the collection. The card cache currently has
      the same effect, sort of, right?
      <div><br>
      </div>
      <div>igor<br>
        <div><br>
          <div>
            <div>On Jun 28, 2013, at 12:26 PM, John Cuthbertson <<a moz-do-not-send="true" href="mailto:john.cuthbertson@oracle.com">john.cuthbertson@oracle.com</a>>
              wrote:</div>
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              <div text="#000000" bgcolor="#FFFFFF"> Hi Igor,<br>
                <br>
                <div class="moz-cite-prefix">On 6/28/2013 9:47 AM, Igor
                  Veresov wrote:<br>
                </div>
                <blockquote cite="mid:0797C324-BE96-4A60-9154-B23FED4B6A43@gmail.com" type="cite">
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                  <base href="x-msg://368/"><br>
                  <div>
                    <div>On Jun 28, 2013, at 7:08 AM, "Doerr, Martin"
                      <<a moz-do-not-send="true" href="mailto:martin.doerr@sap.com">martin.doerr@sap.com</a>>

                      wrote:</div>
                    <br class="Apple-interchange-newline">
                    <blockquote type="cite">
                      <div link="blue" vlink="purple" style="font-family: Helvetica; font-size:
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                        text-align: -webkit-auto; text-indent: 0px;
                        text-transform: none; white-space: normal;
                        widows: 2; word-spacing: 0px;
                        -webkit-text-size-adjust: auto;
                        -webkit-text-stroke-width: 0px; " lang="DE">
                        <div class="WordSection1" style="page:
                          WordSection1; ">
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US">Hi
                              Igor,<o:p></o:p></span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US"> </span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US">we
                              didnít find an easy and feasible way to
                              ensure the ordering, either.<o:p></o:p></span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US">Grabbing

                              the buffers and cleaning the cards at
                              safepoints might be the best solution.</span></div>
                        </div>
                      </div>
                    </blockquote>
                    <div><br>
                    </div>
                    <div>Would anybody from the G1 team like to think
                      about that?</div>
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                <br>
                I've been thinking about this issue on an off for the
                last few weeks when I get the time. I mentioned it to
                Vladimir a couple of times to get his input.<br>
                <br>
                <blockquote cite="mid:0797C324-BE96-4A60-9154-B23FED4B6A43@gmail.com" type="cite">
                  <div>
                    <blockquote type="cite">
                      <div link="blue" vlink="purple" style="font-family: Helvetica; font-size:
                        medium; font-style: normal; font-variant:
                        normal; font-weight: normal; letter-spacing:
                        normal; line-height: normal; orphans: 2;
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                        text-transform: none; white-space: normal;
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                        <div class="WordSection1" style="page:
                          WordSection1; ">
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US"><o:p></o:p></span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US"> </span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US">Maybe

                              removing the barrier that flushes the
                              store to the cardtable makes the problem
                              more likely to occur.<o:p></o:p></span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US">I
                              guess the purpose of the barrier was
                              exactly to avoid this problem<o:p></o:p></span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US">(which

                              should be working perfectly if the post
                              barriers had StoreLoad barriers, too).<o:p></o:p></span></div>
                          <div style="margin: 0cm 0cm 0.0001pt;
                            font-size: 12pt; font-family: 'Times New
                            Roman', serif; "><span style="font-size:
                              11pt; font-family: Calibri, sans-serif;
                              color: rgb(31, 73, 125); " lang="EN-US"> </span></div>
                        </div>
                      </div>
                    </blockquote>
                    <br>
                    <div>Yeah, but like you noted that would have a
                      horrific effect on performance. So, it's probably
                      best to bunch the work up to at least eliminate
                      the need of extra work when, say, you're looping
                      and storing to a limited working set (G1 uses the
                      cardtable basically for that purpose). The
                      safepoint approach will likely require more memory
                      for buffers and the load will be spiky, and if the
                      collection were to happen right after we grabbed
                      the buffers the collector will have to process all
                      of them which is not going to work well for
                      predictability. But nothing better comes to mind
                      at this point.</div>
                    <div>Btw, there are already periodic safepoints to
                      do bias locking revocations, so may be it would
                      make sense to piggyback on that.  <br>
                    </div>
                  </div>
                </blockquote>
                <br>
                Piggy backing on all the other safepoint operations
                might work if they happen frequently enough but I don't
                know if that 's the case. And as you, even then there
                will be times where we haven't had a safepoint for a
                while and will have a ton of buffers to process at the
                start of the pause.<br>
                <br>
                It might be worth adding a suitable memory barrier to
                the G1 post write barrier and evaluating the throughput
                hit.<br>
                <br>
                JohnC<br>
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