<div dir="ltr">Hi Serkan,<div><br></div><div>Taobao developed something called "GCIH" (GC-Invisible Heap), which is also an off-heap solution, that might be similar to what you're trying to do. I was a part of the effort when I worked there.</div>
<div><br></div><div>The JVM part of source code of a very very early version of the GCIH is available here: <a href="http://jvm.taobao.org/images/4/49/Jvm_gcih.patch">http://jvm.taobao.org/images/4/49/Jvm_gcih.patch</a></div>
<div><br></div><div>We record the high-water mark whenever we touch a PermGen object when moving objects into GCIH:</div><div><br></div><div><div>+ if (p->is_klass() || p->is_perm()) {</div><div>+ if (GCInvisibleHeap::_top_klass_addr < p) {</div>
<div>+ GCInvisibleHeap::_top_klass_addr = p;</div><div>+ }</div><div>+ return;</div><div>+ }</div></div><div><br></div><div>And then there were multiple ways to do things. In this version of GCIH we would traverse all objects in GCIH to fixup their klass pointers after the PermGen has been compacted. There was another version that would simply prevent the GC from compacting the part of PermGen below our high-water mark. I'm not sure how it evolved after I left, but the implementation is much more stable now, so they might have come up with a better way to do it.</div>
<div><br></div><div>Nonetheless, all these solutions require customizing the JVM internals, which might not be the thing you want to do.</div><div><br></div><div>If you target your off-heap solution to only Java 8 or above, and only targeting HotSpot, however, then you don't have to worry about metadata objects moving around (at least for now). That's because the PermGen is removed from GC and moved into a piece of native memory called "Metaspace", so they're not subject to GC compaction anymore.</div>
<div><br></div><div>I should mention both JRockit and IBM J9 don't have a PermGen even before Java 8, and their metadata objects are not subject to compaction either.</div><div>I guess you're trying to be JVM-implementation-agnostic here, but since not all JVMs do compaction, and not all JVMs support object pinning, there's no cross-JVM compatible API that allows you to prevent metadata object from being compacted.</div>
<div><br></div><div>- Kris</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Sun, Jun 1, 2014 at 11:42 AM, serkan özal <span dir="ltr"><<a href="mailto:serkanozal86@hotmail.com" target="_blank">serkanozal86@hotmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div><div dir="ltr"><div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">Hi all,</span><br style="line-height:21.299999237060547px;color:rgb(68,68,68);font-size:15px"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px"><br>
</span></div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">I am not sure that target of mail is this group or not but I don't know better one for asking :)</span><br style="line-height:21.299999237060547px;color:rgb(68,68,68);font-size:15px">
<span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px"><br></span></div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">I am currently working on an OffHeap solution and I have a problem with "Compact" phase of GC.As I see at "Compact" phase, location of classes may be changed. I tried class pinning with JNI by "NewGlobalRef" method but it doesn't prevent compacting. As I understood, it only hides object from garbage collected.</span><br style="line-height:21.299999237060547px;color:rgb(68,68,68);font-size:15px">
<span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">In brief, is there any way to prevent compacting of any specific class defition (or object) at GC cycle?Is there any bit, offset or field (such as mark_oop) in object header to prevent compacting of fully from GC for any specific object or class?</span><br style="line-height:21.299999237060547px;color:rgb(68,68,68);font-size:15px">
<span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px"><br></span></div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">Thanks in advance.</span><br style="line-height:21.299999237060547px;color:rgb(68,68,68);font-size:15px">
<span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px"><br></span></div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">--</span><br style="line-height:21.299999237060547px;color:rgb(68,68,68);font-size:15px">
<span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px"><br></span></div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px">Serkan ÖZAL</span> </div>
</div><div dir="ltr"><span style="color:rgb(68,68,68);font-size:15px;line-height:21.299999237060547px"><br></span></div> </div></div>
</blockquote></div><br></div>