OK. Your further improvement idea sounds promising.
dl
On 2/7/2014 3:55 PM, Oleg Mazurov wrote:
The cost might be minimal but that would be a move in the wrong
direction in my opinion.
A larger problem is that BufferBlobs being just placeholders for
dynamic code should not be
reported via JVMTI at all: when they are created they usually contain
no executable code and
actual objects placed into such blobs are reported separately (that
wasn't true for vtable/itable
stubs before this fix). That the same address, that of a BufferBlob
and its first object, could be
reported twice is revealed by a comment to a loop that follows the
problematic comparison:
src/share/vm/prims/jvmtiCodeBlobEvents.cpp:
124 // exclude VtableStubs, which are processed separately
125 if (cb->is_buffer_blob() && strcmp(cb->name(), "vtable chunks")
== 0) {
126 return;
127 }
128
129 // check if this starting address has been seen already - the
130 // assumption is that stubs are inserted into the list before the
131 // enclosing BufferBlobs.
132 address addr = cb->code_begin();
133 for (int i=0; i<_global_code_blobs->length(); i++) {
134 JvmtiCodeBlobDesc* scb = _global_code_blobs->at(i);
135 if (addr == scb->code_begin()) {
136 return;
137 }
138 }
I believe that now that the vtable stub problem is fixed the need for
that check is gone
and both the strcmp() call and the following loop could be removed
altogether, thus stopping
further processing for *any* BufferBlob and avoiding a O(n^2) overhead
they were causing.
The scope of that change is much larger than the original problem
entailed and would require
not just additional ad hoc testing on the JMTI consumer side but also
thorough statical analysis
for all BufferBlob uses in the VM code.
In fact, I was going to file a linked JIRA issue on that further
improvement and if my idea for
it holds true there would be no need for a new BufferBlob subtype.
-- Oleg
On 2/7/2014 1:06 PM, Dean Long wrote:
What's the cost for adding a new BufferBlob subtype? We already have
AdapterBlob and MethodHandlesAdapterBlob.
dl
On 2/6/2014 3:52 PM, Oleg Mazurov wrote:
My understanding was that a buffer blob was just that - a buffer.
Could potentially contain code fragments of different kinds.
Thus, is_buffer_blob() was the closest type available. Agree that a
dependency on its name is not reliable, though testing
will reveal if the condition turns false for "vtable chunks" due to
a name change (I had to deal with that particular test, Serguei
should be able to identify it). Adding a comment to where the name
is defined (vtableStubs.cpp) that such a dependency exists
is a good idea.
Thanks,
-- Oleg
On Feb 6, 2014, at 3:32 PM, Coleen Phillimore wrote:
Hi, I clicked on this a couple times. It seems okay but isn't there
a safer way to identify code blobs that are vtable stubs, without
looking at the name (which can change in while creating it). A
comment at least when you create "vtable chunks" would be good.
It seems that someone might want to rename it "vtable or itable
buffers", or something like that.
thanks,
Coleen
On 2/6/14 6:17 PM, serguei.spit...@oracle.com wrote:
Runtime team,
This fix was reviewed by Vladimir K. and me.
Just wanted to make sure if you would like to review it as well.
If not, then I will push it.
Thanks,
Serguei
On 2/3/14 2:17 PM, serguei.spit...@oracle.com wrote:
Please, review the fix for:
https://bugs.openjdk.java.net/browse/JDK-8025841
Open webrev:
http://cr.openjdk.java.net/~sspitsyn/webrevs/2014/hotspot/omazurov/8025841-JVMTI-vtbl.1
Summary:
The fix contributed by Oleg Mazurov to improve profiling data
quality.
It moves the "vtable stub" dynamic code notification to the
right place.
I've already reviewed the fix, and it looks good to me.
Bug report description:
"JVMTI_EVENT_DYNAMIC_CODE_GENERATED for "vtable stub" gets
scheduled when
a new chunk of memory for subsequent vtable and itable stubs
is allocated.
That chunk is uninitialized (contains zeros or garbage)
although due to the fact
that the actual event delivery is deferred, at least one
vtable comes out right.
This event should describe an individual vtable/itable stub
(base address and size)
and only after it's been created (memory is actually
populated with code).
Where VM diagnostic messages about vtable/itable stubs are
issued upon
-XX:+PrintAdapterHandlers appears exactly the right place
for JVMTI events as well.
Getting vtables/itables right is important in the context of
performance analysis as
that dynamically generated code may accumulate quite
noticeable CPU time
(especially itabes), sometimes larger than the actual Java
methods called."
Testing:
Oleg tested it in the Oracle Studio Performance Analyzer
environment.
nsk.jvmti, nsk.jdi, nsk.jdwp,
In progress: Jtreg com/sun/jdi, java/lang/instrument
Thanks,
Serguei
