RFR: 8227269: Slow class loading when running JVM in debug mode
serguei.spitsyn at oracle.com
serguei.spitsyn at oracle.com
Tue Mar 24 21:46:33 UTC 2020
On 3/24/20 14:39, Chris Plummer wrote:
> On 3/24/20 1:45 PM, Roman Kennke wrote:
>>>>> I assume JVMTI maintains separate tagging data for each agent so
>>>>> having
>>>>> two agents doing tagging won't result in confusion. I didn't actually
>>>>> find this in the spec. Would be nice to confirm that it is the case.
>>>>> However, your implementation does seem to conflict with other uses of
>>>>> tagging in the debug agent:
>>>> The tagging data is per-jvmtiEnv. We create and use our own env
>>>> (private
>>>> to class-tracking), so this wouldn't conflict with other uses of tags.
>>>> Could it be a problem that we have a single trackingEnv per JVM,
>>>> though?
>>>> /me scratches head.
>>> Ok. This is an area I'm not familiar with, but the spec does say:
>>>
>>> "Each call to GetEnv creates a new JVM TI connection and thus a new JVM
>>> TI environment."
>>>
>>> So it looks like what you are doing should be ok. I still think you
>>> have
>>> a bug where you are not deallocating signatures of classes that are
>>> unloaded. If you think otherwise please point out where this is done.
>> Signatures that make it out of processUnloading() are deallocated in
>> eventHandler.c, in synthesizeUnload(), right after it has been used.
>>
>> http://hg.openjdk.java.net/jdk/jdk/file/b9562cc25fc0/src/jdk.jdwp.agent/share/native/libjdwp/eventHandler.c#l527
>>
> Ok. Good to know. Not the best of designs, but that's not your fault.
> I'll make another pass over the changes, but I think in general it
> looks good. I don't think I've seen another reviewer yet, so hopefully
> someone jumps in.
As I understand, Roman already resolved my previous comments.
So, I will do another pass for v6.
Thanks,
Serguei
>
> Chris
>> Pending signatures on debug-agent-disconnect are deallocated in
>> classTrack.c, in the reset() routine.
>>
>> Thanks,
>> Roman
>>
>>> thanks,
>>>
>>> Chris
>>>>> What would cause classTrack_addPreparedClass() to be called for a
>>>>> Class
>>>>> you've already seen? I don't understand the need for the "tag != 0l"
>>>>> check.
>>>> It's probably not needed, may be a left-over from previous
>>>> installments
>>>> of this implementation. I will check it, and turn into an assert or
>>>> so.
>>>>
>>>> Thanks,
>>>> Roman
>>>>
>>>>> thanks,
>>>>>
>>>>> Chris
>>>>>
>>>>> On 3/20/20 12:52 PM, Chris Plummer wrote:
>>>>>> On 3/20/20 8:30 AM, Roman Kennke wrote:
>>>>>>> I believe I came up with a much simpler solution that also
>>>>>>> solves the
>>>>>>> problems of the existing one, and the ones I proposed earlier.
>>>>>>>
>>>>>>> It turns out that we can take advantage of the fact that we can use
>>>>>>> *anything* as tags in JVMTI, even pointers to stuff (this is
>>>>>>> explicitely
>>>>>>> mentioned in the JVMTI spec). This means we can simply stick a
>>>>>>> pointer
>>>>>>> to the signature of a class into the tag, and pull it out again
>>>>>>> when we
>>>>>>> get notified that the class gets unloaded.
>>>>>>>
>>>>>>> This means we don't need an extra data-structure to keep track of
>>>>>>> classes and signatures, and it also makes the story around locking
>>>>>>> *much* simpler. Performance-wise this is O(1), i.e. no scanning
>>>>>>> of all
>>>>>>> classes needed (as in the current implementation) and no
>>>>>>> searching of
>>>>>>> table needed (like in my previous attempts).
>>>>>>>
>>>>>>> Please review this new revision:
>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.06/
>>>>>> I'll have a look at this.
>>>>>>> (Notice that there still appears to be a performance bottleneck
>>>>>>> with
>>>>>>> class-unloading when an actual debugger is attached. This
>>>>>>> doesn't seem
>>>>>>> to be related to the classTrack.c implementation though, but looks
>>>>>>> like
>>>>>>> a consequence of getting all those class-unload notifications
>>>>>>> over the
>>>>>>> wire. My testcase generates 1000s of them, and it's clogging up the
>>>>>>> buffers.)
>>>>>> At least this is only a one-shot hit when the classes are unloaded,
>>>>>> and the performance hit is based on the number of classes being
>>>>>> unloaded. The main issue is happening every GC, and is O(n) where
>>>>>> n is
>>>>>> the number of loaded classes.
>>>>>>> I am not sure why jdb needs to enable class-unload listener
>>>>>>> always. A
>>>>>>> simple hack disables it, and performance is brilliant, even when
>>>>>>> jdb is
>>>>>>> attached:
>>>>>>> http://cr.openjdk.java.net/~rkennke/disable-jdk-class-unload.patch
>>>>>> This is JDI, not jdb. It looks like it needs ClassUnload events
>>>>>> so it
>>>>>> can maintain typesBySignature, which is used by public APIs like
>>>>>> allClasses(). So we have caching of loaded classes both in the debug
>>>>>> agent and in JDI.
>>>>>>
>>>>>> Chris
>>>>>>> But this is not in the scope of this bug.)
>>>>>>>
>>>>>>> Roman
>>>>>>>
>>>>>>>
>>>>>>> On 3/16/20 8:05 AM, serguei.spitsyn at oracle.com wrote:
>>>>>>>> Sorry, forgot to complete my comments at the end (see below).
>>>>>>>>
>>>>>>>>
>>>>>>>> On 3/15/20 23:57, serguei.spitsyn at oracle.com wrote:
>>>>>>>>> Hi Roman,
>>>>>>>>>
>>>>>>>>> Thank you for the update and sorry for the latency in review.
>>>>>>>>>
>>>>>>>>> Some comments are below.
>>>>>>>>>
>>>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.05/src/jdk.jdwp.agent/share/native/libjdwp/classTrack.c.frames.html
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> 87 cbTrackingObjectFree(jvmtiEnv* jvmti_env, jlong tag)
>>>>>>>>> 88 {
>>>>>>>>> 89 debugMonitorEnter(deletedSignatureLock);
>>>>>>>>> 90 if (currentClassTag == -1) {
>>>>>>>>> 91 // Class tracking not initialized, nobody's interested
>>>>>>>>> 92 debugMonitorExit(deletedSignatureLock);
>>>>>>>>> 93 return;
>>>>>>>>> 94 }
>>>>>>>>> Just a question:
>>>>>>>>> Q1: Should the ObjectFree events be disabled for the
>>>>>>>>> jvmtiEnv
>>>>>>>>> that does
>>>>>>>>> the class tracking if class tracking has not been
>>>>>>>>> initialized?
>>>>>>>>>
>>>>>>>>> 70 static jlong currentClassTag; I'm thinking if the name is
>>>>>>>>> better to
>>>>>>>>> be something like: lastClassTag or highestClassTag.
>>>>>>>>>
>>>>>>>>> 99 KlassNode* klass = *klass_ptr;
>>>>>>>>> 100 102 while (klass != NULL && klass->klass_tag != tag) { 103
>>>>>>>>> klass_ptr = &klass->next; 104 klass = *klass_ptr;
>>>>>>>>> 105 } 106 if (klass != NULL || klass->klass_tag != tag) { //
>>>>>>>>> klass not
>>>>>>>>> found - ignore.
>>>>>>>>> 107 debugMonitorExit(deletedSignatureLock);
>>>>>>>>> 108 return;
>>>>>>>>> 109 }
>>>>>>>>> It seems to me, something is wrong in the condition at L106
>>>>>>>>> above.
>>>>>>>>> Should it be? :
>>>>>>>>> if (klass == NULL || klass->klass_tag != tag)
>>>>>>>>>
>>>>>>>>> Otherwise, how can the second check ever work correctly as
>>>>>>>>> the
>>>>>>>>> return
>>>>>>>>> will always happen when (klass != NULL)?
>>>>>>>>>
>>>>>>>>> There are several places in this file with the the indent:
>>>>>>>>> 90 if (currentClassTag == -1) {
>>>>>>>>> 91 // Class tracking not initialized, nobody's interested
>>>>>>>>> 92 debugMonitorExit(deletedSignatureLock);
>>>>>>>>> 93 return;
>>>>>>>>> 94 }
>>>>>>>>> ...
>>>>>>>>> 152 if (currentClassTag == -1) {
>>>>>>>>> 153 // Class tracking not initialized yet, nobody's interested
>>>>>>>>> 154 debugMonitorExit(deletedSignatureLock);
>>>>>>>>> 155 return;
>>>>>>>>> 156 }
>>>>>>>>> ...
>>>>>>>>> 161 if (error != JVMTI_ERROR_NONE) {
>>>>>>>>> 162 EXIT_ERROR(error, "Unable to GetTag with class trackingEnv");
>>>>>>>>> 163 }
>>>>>>>>> 164 if (tag != 0l) {
>>>>>>>>> 165 debugMonitorExit(deletedSignatureLock);
>>>>>>>>> 166 return; // Already added
>>>>>>>>> 167 }
>>>>>>>>> ...
>>>>>>>>> 281 cleanDeleted(void *signatureVoid, void *arg)
>>>>>>>>> 282 {
>>>>>>>>> 283 char* sig = (char*)signatureVoid;
>>>>>>>>> 284 jvmtiDeallocate(sig);
>>>>>>>>> 285 return JNI_TRUE;
>>>>>>>>> 286 }
>>>>>>>>> ...
>>>>>>>>> 291 void
>>>>>>>>> 292 classTrack_reset(void)
>>>>>>>>> 293 {
>>>>>>>>> 294 int idx;
>>>>>>>>> 295 debugMonitorEnter(deletedSignatureLock);
>>>>>>>>> 296
>>>>>>>>> 297 for (idx = 0; idx < CT_SLOT_COUNT; ++idx) {
>>>>>>>>> 298 KlassNode* node = table[idx];
>>>>>>>>> 299 while (node != NULL) {
>>>>>>>>> 300 KlassNode* next = node->next;
>>>>>>>>> 301 jvmtiDeallocate(node->signature);
>>>>>>>>> 302 jvmtiDeallocate(node);
>>>>>>>>> 303 node = next;
>>>>>>>>> 304 }
>>>>>>>>> 305 }
>>>>>>>>> 306 jvmtiDeallocate(table);
>>>>>>>>> 307
>>>>>>>>> 308 bagEnumerateOver(deletedSignatureBag, cleanDeleted, NULL);
>>>>>>>>> 309 bagDestroyBag(deletedSignatureBag);
>>>>>>>>> 310
>>>>>>>>> 311 currentClassTag = -1;
>>>>>>>>> 312
>>>>>>>>> 313
>>>>>>>>> (void)JVMTI_FUNC_PTR(trackingEnv,DisposeEnvironment)(trackingEnv);
>>>>>>>>>
>>>>>>>>> 314 trackingEnv = NULL;
>>>>>>>>> 315
>>>>>>>>> 316 debugMonitorExit(deletedSignatureLock);
>>>>>>>>>
>>>>>>>>> Could you, please, fix several comments below?
>>>>>>>>> 63 * The JVMTI tracking env to keep track of klass tags, for
>>>>>>>>> class-unloads
>>>>>>>>> The comma is not needed.
>>>>>>>>> Would it better to replace: klass tags => klass_tag's ?
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> 73 * Lock to keep table, currentClassTag and deletedSignatureBag
>>>>>>>>> consistent
>>>>>>>>> Maybe: Lock to guard ... or lock to keep integrity of ...
>>>>>>>>>
>>>>>>>>> 84 * Callback when classes are freed, Finds the signature and
>>>>>>>>> remembers it in deletedSignatureBag. Would be better to use words
>>>>>>>>> like
>>>>>>>>> "store" or "record", "Find" should not start from capital letter:
>>>>>>>>> Invoke the callback when classes are freed, find and record the
>>>>>>>>> signature in deletedSignatureBag.
>>>>>>>>>
>>>>>>>>> 96 // Find deleted KlassNode 133 // Class tracking not
>>>>>>>>> initialized,
>>>>>>>>> nobody's interested 153 // Class tracking not initialized yet,
>>>>>>>>> nobody's interested 158 /* Check this is not a duplicate */
>>>>>>>>> Missed dot
>>>>>>>>> at the end. 106 if (klass != NULL || klass->klass_tag != tag)
>>>>>>>>> { //
>>>>>>>>> klass not found - ignore. In opposite, dot is not needed as the
>>>>>>>>> comment does not start from a capital letter. 111 // At this
>>>>>>>>> point we
>>>>>>>>> have the KlassNode corresponding to the tag
>>>>>>>>> 112 // in klass, and the pointer to it in klass_node.
>>>>>>>> The comment above can be better. Maybe, something like:
>>>>>>>> " At this point, we found the KlassNode matching the klass
>>>>>>>> tag(and it is
>>>>>>>> linked).
>>>>>>>>
>>>>>>>>> 113 // Remember the unloaded signature.
>>>>>>>> Better: Record the signature of the unloaded class and
>>>>>>>> unlink it.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Serguei
>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Serguei
>>>>>>>>>
>>>>>>>>> On 3/9/20 05:39, Roman Kennke wrote:
>>>>>>>>>> Hello all,
>>>>>>>>>>
>>>>>>>>>> Can I please get reviews of this change? In the meantime, we've
>>>>>>>>>> done
>>>>>>>>>> more testing and also field-/torture-testing by a customer
>>>>>>>>>> who is
>>>>>>>>>> happy
>>>>>>>>>> now. :-)
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>> Roman
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> Hi Serguei,
>>>>>>>>>>>
>>>>>>>>>>> Thanks for reviewing!
>>>>>>>>>>>
>>>>>>>>>>> I updated the patch to reflect your suggestions, very good!
>>>>>>>>>>> It also includes a fix to allow re-connecting an agent after
>>>>>>>>>>> disconnect,
>>>>>>>>>>> namely move setup of the trackingEnv and deletedSignatureBag to
>>>>>>>>>>> _activate() to ensure have those structures after re-connect.
>>>>>>>>>>>
>>>>>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.05/
>>>>>>>>>>>
>>>>>>>>>>> Let me know what you think!
>>>>>>>>>>> Roman
>>>>>>>>>>>
>>>>>>>>>>>> Hi Roman,
>>>>>>>>>>>>
>>>>>>>>>>>> Thank you for taking care about this scalability issue!
>>>>>>>>>>>>
>>>>>>>>>>>> I have a couple of quick comments.
>>>>>>>>>>>>
>>>>>>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.04/src/jdk.jdwp.agent/share/native/libjdwp/classTrack.c.frames.html
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> 72 /*
>>>>>>>>>>>> 73 * Lock to protect deletedSignatureBag
>>>>>>>>>>>> 74 */
>>>>>>>>>>>> 75 static jrawMonitorID deletedSignatureLock; 76 77 /*
>>>>>>>>>>>> 78 * A bag containing all the deleted classes' signatures.
>>>>>>>>>>>> Must be
>>>>>>>>>>>> accessed under
>>>>>>>>>>>> 79 * deletedTagLock,
>>>>>>>>>>>> 80 */
>>>>>>>>>>>> 81 struct bag* deletedSignatureBag;
>>>>>>>>>>>>
>>>>>>>>>>>> The comments contradict to each other.
>>>>>>>>>>>> I guess, the lock name at line 79 has to be
>>>>>>>>>>>> deletedSignatureLock
>>>>>>>>>>>> instead of deletedTagLock.
>>>>>>>>>>>> Also, comma at the end must be replaced with dot.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> 101 // Tag not found? Ignore.
>>>>>>>>>>>> 102 if (klass == NULL) {
>>>>>>>>>>>> 103 debugMonitorExit(deletedSignatureLock);
>>>>>>>>>>>> 104 return;
>>>>>>>>>>>> 105 }
>>>>>>>>>>>> 106
>>>>>>>>>>>> 107 // Scan linked-list.
>>>>>>>>>>>> 108 jlong found_tag = klass->klass_tag;
>>>>>>>>>>>> 109 while (klass != NULL && found_tag != tag) {
>>>>>>>>>>>> 110 klass_ptr = &klass->next;
>>>>>>>>>>>> 111 klass = *klass_ptr;
>>>>>>>>>>>> 112 found_tag = klass->klass_tag;
>>>>>>>>>>>> 113 }
>>>>>>>>>>>> 114
>>>>>>>>>>>> 115 // Tag not found? Ignore.
>>>>>>>>>>>> 116 if (found_tag != tag) {
>>>>>>>>>>>> 117 debugMonitorExit(deletedSignatureLock);
>>>>>>>>>>>> 118 return;
>>>>>>>>>>>> 119 }
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> The code above can be simplified, so that the lines
>>>>>>>>>>>> 101-105
>>>>>>>>>>>> are not
>>>>>>>>>>>> needed anymore.
>>>>>>>>>>>> It can be something like this:
>>>>>>>>>>>>
>>>>>>>>>>>> // Scan linked-list.
>>>>>>>>>>>> while (klass != NULL && klass->klass_tag != tag) {
>>>>>>>>>>>> klass_ptr = &klass->next;
>>>>>>>>>>>> klass = *klass_ptr;
>>>>>>>>>>>> }
>>>>>>>>>>>> if (klass == NULL || klass->klass_tag != tag) { // klass not
>>>>>>>>>>>> found - ignore.
>>>>>>>>>>>> debugMonitorExit(deletedSignatureLock);
>>>>>>>>>>>> return;
>>>>>>>>>>>> }
>>>>>>>>>>>>
>>>>>>>>>>>> It will take more time when I get a chance to look at the
>>>>>>>>>>>> rest.
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> Thanks,
>>>>>>>>>>>> Serguei
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> On 12/21/19 13:24, Roman Kennke wrote:
>>>>>>>>>>>>> Here comes an update that resolves some races that happen
>>>>>>>>>>>>> when
>>>>>>>>>>>>> disconnecting an agent. In particular, we need to take the
>>>>>>>>>>>>> lock on
>>>>>>>>>>>>> basically every operation, and also need to check whether
>>>>>>>>>>>>> or not
>>>>>>>>>>>>> class-tracking is active and return an appropriate result
>>>>>>>>>>>>> (e.g.
>>>>>>>>>>>>> an empty
>>>>>>>>>>>>> list) when we're not.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Updated webrev:
>>>>>>>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.04/
>>>>>>>>>>>>>
>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>> Roman
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>> So, here comes the O(1) implementation:
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> - Whenever a class is 'prepared', it is registered with a
>>>>>>>>>>>>>> tag,
>>>>>>>>>>>>>> and we
>>>>>>>>>>>>>> set-up a listener to get notified when it is unloaded.
>>>>>>>>>>>>>> - Prepared classes are kept in a datastructure that is a
>>>>>>>>>>>>>> table, which
>>>>>>>>>>>>>> each entry being the head of a linked-list of KlassNode*.
>>>>>>>>>>>>>> The
>>>>>>>>>>>>>> table is
>>>>>>>>>>>>>> indexed by tag % slot-count, and then simply prepend the new
>>>>>>>>>>>>>> KlassNode*.
>>>>>>>>>>>>>> This is O(1) operation.
>>>>>>>>>>>>>> - When we get notified of unloading a class, we look up the
>>>>>>>>>>>>>> signature of
>>>>>>>>>>>>>> the reported tag in that table, and remember it in a bag.
>>>>>>>>>>>>>> The
>>>>>>>>>>>>>> KlassNode*
>>>>>>>>>>>>>> is then unlinked from the table and deallocated. This is
>>>>>>>>>>>>>> ~O(1)
>>>>>>>>>>>>>> operation
>>>>>>>>>>>>>> too, depending on the depth of the table. In my testcase
>>>>>>>>>>>>>> which
>>>>>>>>>>>>>> hammered
>>>>>>>>>>>>>> the code with class-loads and unloads, I usually see
>>>>>>>>>>>>>> depths of
>>>>>>>>>>>>>> like 2-3,
>>>>>>>>>>>>>> but not usually more. It should be ok.
>>>>>>>>>>>>>> - when processUnloads() gets called, we simply hand out that
>>>>>>>>>>>>>> bag, and
>>>>>>>>>>>>>> allocate a new one.
>>>>>>>>>>>>>> - I also added cleanup-code in classTrack_reset() to avoid
>>>>>>>>>>>>>> leaking the
>>>>>>>>>>>>>> signatures and KlassNode* etc when debug agent gets detached
>>>>>>>>>>>>>> and/or
>>>>>>>>>>>>>> re-attached (was missing before).
>>>>>>>>>>>>>> - I also added locks around data-structure-manipulation (was
>>>>>>>>>>>>>> missing
>>>>>>>>>>>>>> before).
>>>>>>>>>>>>>> - Also, I only activate this whole process when an actual
>>>>>>>>>>>>>> listener gets
>>>>>>>>>>>>>> registered on EI_GC_FINISH. This seems to happen right when
>>>>>>>>>>>>>> attaching a
>>>>>>>>>>>>>> jdb, not sure why jdb does that though. This may be
>>>>>>>>>>>>>> something
>>>>>>>>>>>>>> to improve
>>>>>>>>>>>>>> in the future?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> In my tests, the performance of class-tracking itself looks
>>>>>>>>>>>>>> really good.
>>>>>>>>>>>>>> The bottleneck now is clearly actual synthesizing the
>>>>>>>>>>>>>> class-unload
>>>>>>>>>>>>>> events. I don't see how this can be helped when the debug
>>>>>>>>>>>>>> agent asks for it?
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Updated webrev:
>>>>>>>>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.03/
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Please let me know what you think of it.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>> Roman
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Alright, the perfectionist in me got me. I am implementing
>>>>>>>>>>>>>>> the even more
>>>>>>>>>>>>>>> efficient ~O(1) class tracking. Please hold off
>>>>>>>>>>>>>>> reviewing for
>>>>>>>>>>>>>>> now.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Thanks,Roman
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Hi Chris,
>>>>>>>>>>>>>>>>> I'll have a look at this, although it might not be for a
>>>>>>>>>>>>>>>>> few days. In
>>>>>>>>>>>>>>>>> the meantime, maybe you can describe your new
>>>>>>>>>>>>>>>>> implementation in
>>>>>>>>>>>>>>>>> classTrack.c so it's easier to look through the changes.
>>>>>>>>>>>>>>>> Sure.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> The purpose of this class-tracking is to be able to
>>>>>>>>>>>>>>>> determine the
>>>>>>>>>>>>>>>> signatures of unloaded classes when GC/class-unloading
>>>>>>>>>>>>>>>> happened, so that
>>>>>>>>>>>>>>>> we can generate the appropriate JDWP event.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> The current implementation does so by maintaining a
>>>>>>>>>>>>>>>> table of
>>>>>>>>>>>>>>>> currently
>>>>>>>>>>>>>>>> prepared classes by building that table when classTrack is
>>>>>>>>>>>>>>>> initialized,
>>>>>>>>>>>>>>>> and then add new classes whenever a class gets loaded.
>>>>>>>>>>>>>>>> When
>>>>>>>>>>>>>>>> unloading
>>>>>>>>>>>>>>>> occurs, that cache is rebuilt into a new table, and
>>>>>>>>>>>>>>>> compared
>>>>>>>>>>>>>>>> with the
>>>>>>>>>>>>>>>> old table, and whatever is in the old, but not in the new
>>>>>>>>>>>>>>>> table gets
>>>>>>>>>>>>>>>> returned. The problem is that when GCs happen frequently
>>>>>>>>>>>>>>>> and/or many
>>>>>>>>>>>>>>>> classes get loaded+unloaded, this amounts to
>>>>>>>>>>>>>>>> O(classCount*gcCount)
>>>>>>>>>>>>>>>> complexity.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> The new implementation keeps a linked-list of prepared
>>>>>>>>>>>>>>>> classes, and also
>>>>>>>>>>>>>>>> tracks unloads via the listener cbTrackingObjectFree().
>>>>>>>>>>>>>>>> Whenever an
>>>>>>>>>>>>>>>> unload/GC occurs, the list of prepared classes is scanned,
>>>>>>>>>>>>>>>> and classes
>>>>>>>>>>>>>>>> that are also in the deletedTagBag are unlinked (thus
>>>>>>>>>>>>>>>> maintaining the
>>>>>>>>>>>>>>>> prepared-classes-list) and its signature put in the list
>>>>>>>>>>>>>>>> that gets returned.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> The implementation is not perfect. In order to determine
>>>>>>>>>>>>>>>> whether or not
>>>>>>>>>>>>>>>> a class is unloaded, it needs to scan the deletedTagBag.
>>>>>>>>>>>>>>>> That process is
>>>>>>>>>>>>>>>> therefore still O(unloadedClassCount). The assumption here
>>>>>>>>>>>>>>>> is that
>>>>>>>>>>>>>>>> unloadedClassCount << classCount. In my experiments this
>>>>>>>>>>>>>>>> seems to be
>>>>>>>>>>>>>>>> true, and also reasonable to expect.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> (I have some ideas how to improve the implementation to
>>>>>>>>>>>>>>>> ~O(1) but it
>>>>>>>>>>>>>>>> would be considerably more complex: have to maintain a
>>>>>>>>>>>>>>>> (hash)table that
>>>>>>>>>>>>>>>> maps tags -> KlassNode*, unlink them directly upon unload,
>>>>>>>>>>>>>>>> and build the
>>>>>>>>>>>>>>>> unloaded-signatures list there, but I don't currently see
>>>>>>>>>>>>>>>> that it's
>>>>>>>>>>>>>>>> worth the effort).
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> In addition to all that, this process is only activated
>>>>>>>>>>>>>>>> when
>>>>>>>>>>>>>>>> there's an
>>>>>>>>>>>>>>>> actual listener registered for EI_GC_FINISH.
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>> Roman
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> Chris
>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>> On 12/18/19 5:05 AM, Roman Kennke wrote:
>>>>>>>>>>>>>>>>>> Hello all,
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Issue:
>>>>>>>>>>>>>>>>>> https://bugs.openjdk.java.net/browse/JDK-8227269
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I am proposing what amounts to a rewrite of
>>>>>>>>>>>>>>>>>> classTrack.c.
>>>>>>>>>>>>>>>>>> It avoids
>>>>>>>>>>>>>>>>>> throwing away the class cache on GC, and instead keeps
>>>>>>>>>>>>>>>>>> track of
>>>>>>>>>>>>>>>>>> loaded/unloaded classes one-by-one.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> In addition to that, it avoids this whole dance until an
>>>>>>>>>>>>>>>>>> agent
>>>>>>>>>>>>>>>>>> registers interest in EI_GC_FINISH.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Webrev:
>>>>>>>>>>>>>>>>>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.01/
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Testing: manual testing of provided test scenarios and
>>>>>>>>>>>>>>>>>> timing.
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Eg with the testcase provided here:
>>>>>>>>>>>>>>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1751985
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I am getting those numbers:
>>>>>>>>>>>>>>>>>> unpatched: no debug: 84s with debug: 225s
>>>>>>>>>>>>>>>>>> patched: no debug: 85s with debug: 95s
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> I also tested successfully through jdk/submit repo
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Can I please get a review?
>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>>>>>>> Roman
>>>>>>>>>>>>>>>>>>
>>>
>
>
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