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.
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
