RFR: 8227269: Slow class loading when running JVM in debug mode
Chris Plummer
chris.plummer at oracle.com
Wed Mar 25 18:07:48 UTC 2020
Hi Roman,
Regarding the new assert:
105 if (gdata && gdata->assertOn) {
106 // Check this is not already tagged.
107 jlong tag;
108 error = JVMTI_FUNC_PTR(trackingEnv, GetTag)(env, klass, &tag);
109 if (error != JVMTI_ERROR_NONE) {
110 EXIT_ERROR(error, "Unable to GetTag with class
trackingEnv");
111 }
112 JDI_ASSERT(tag == NOT_TAGGED);
113 }
I think you should remove the gdata check. gdata should never be NULL
when you get to this code. If it is ever NULL then there's a bug, and
the check will hide the bug.
Regarding testing, after you do the submit repo testing let me know the
jobID and I'll do additional testing on it.
thanks,
Chris
On 3/25/20 6:00 AM, Roman Kennke wrote:
> Hi Sergei,
>
>> The fix looks pretty clean now.
>> I also like new name of the lock.:)
> Thank you!
>
>> Just one comment below.
>>
>> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.06/src/jdk.jdwp.agent/share/native/libjdwp/classTrack.c.frames.html
>>
>> 110 if (tag != 0l) {
>> 111 return; // Already added
>> 112 }
>>
>> It is better to use a named constant or macro instead.
>> Also, it'd be nice to add a short comment about this value is.
> As I replied to Chris earlier, this whole block can be turned into an
> assert. I also made a constant for the value 0, which should be pretty
> much self-explaining.
>
> http://cr.openjdk.java.net/~rkennke/JDK-8227269/webrev.07/
>
>> How do you test the fix?
> I am using a manual test that is provided in this bug report:
> https://bugzilla.redhat.com/show_bug.cgi?id=1751985
>
> "Script to compare performance of GC with and without debugger, when
> many classes are loaded and classes are being unloaded":
>
> https://bugzilla.redhat.com/attachment.cgi?id=1640688
>
> I am also using this test and manually attach/detach jdb a couple of
> times in a row to check that disconnecting and reconnecting works well
> (this tended to deadlock or crash with an earlier version of the patch,
> and is now looking good).
>
> I am also running tier1 and tier2 tests locally, and as soon as we all
> agree that the fix is reasonable, I will push it to the submit repo. I
> am not sure if any of those tests actually exercise that code, though.
> Let me know if you want me to run any specific tests.
>
> Thank you,
> Roman
>
>
>
>> Thanks,
>> Serguei
>>
>>
>> On 3/20/20 08:30, 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/
>>>
>>> (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.)
>>>
>>> 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
>>>
>>> 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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