On Thu, 15 Aug 2024 14:32:40 GMT, Alan Bateman <[email protected]> wrote:
>> Would it be possible to create a boolean in the EventWriter that indicates
>> if it is associated with a carrier thread or a normal thread (which can
>> never be virtual) and then have two methods.
>>
>> long l = this.carrierThread ? StringPool.addPinnedString(s) :
>> StringPool.addString(s);
>
> Thread.currentThread() has an intrinsic, and isVirtual is just a type check.
> ContinuationSupport.isSupported reads a static final so will disappear once
> compiled. The pattern we are using in other areas is for the pin to return a
> boolean (like David suggested).
I looked into this in more detail. The current suggestion:
mov r10,QWORD PTR [r15+0x388] ; _vthread OopHandle
mov r10,QWORD PTR [r10] ; dereference OopHandle <<--
Thread.currentThread() intrinsic gives 2 instructions
mov r11d,DWORD PTR [r10+0x8] ; InstanceKlass to r11 <-- isVirtual()
mov r10d,r11d ; InstanceKlass to r10
mov r8,QWORD PTR [r10+0x40] ; Load slot in InstanceKlass primary supers
array to r8
movabs r10,0x2d0481a8 ; InstanceKlass for
{metadata('java/lang/BaseVirtualThread')} to r10
cmp r8,r10 ; compare if superklass is
java/lang/BaseVirtualThread
jne 0x0000018571e0baf9 ; 6 instructions for isVirtual() type
check, 8 instructions in total
This gives a prologue of eight instructions.
For JFR, we already have much of this information resolved when loading up the
EventWriter instance using the existing intrinsic getEventWriter(). Hence, we
could extend that to mark the event writer with a field to say if pinning
should be performed. This results in only a two instruction prologue:
test r8d,r8d ; pinVirtualThread?
je 0x0000012580a0f6c9 ; 2 instructions for test
This is an x4 speedup, although slightly less because of an additional store
instruction for loading the event writer.
Further, I looked into the Continuation.pin() and Continuation.unpin() methods.
They are currently not intrinsics, but lend themselves well to
intrinsification. I have created such intrinsics, and the results are quite
good.
Continuation.pin() or Continuation.unpin() without intrinsics = 112
instructions each
Continuation.pin() or Continuation.unpin() with intrinsics = 8 instructions each
This is an x14 speedup for virtual threads.
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PR Review Comment: https://git.openjdk.org/jdk/pull/20588#discussion_r1725145256
