[webkit-dev] VM::setExclusiveThread()

Mark Lam mark.lam at apple.com
Tue Feb 28 10:38:43 PST 2017

I’ve run Speedometer many times on a quiet 13” MacBookPro: removing the use of exclusive thread status does not appear to impact performance in any measurable way.  I also did some measurements on a microbenchmark locking and unlocking the JSLock using a JSLockHolder in a loop.  The microbenchmark shows that removing exclusive thread status results in the locking and unlocking operation increasing by up to 20%.

Given that locking and unlocking the JSLock is a very small fraction of the work done in a webpage, it’s not surprising that the 20% increase in time for the lock and unlock operation is not measurable in Speedometer.  Note also that the 20% only impacts WebCore which uses the exclusive thread status.  For all other clients of JSC (which never uses exclusive thread status), it may actually be faster to have exclusive thread checks removed (simply due to that code doing less work).

I’ll put up a patch to remove the use of exclusive thread status.  This will simplify the code and make it easier to move forward with new features.


> On Feb 24, 2017, at 9:01 PM, Filip Pizlo <fpizlo at apple.com> wrote:
> Seems like if the relevant benchmarks (speedometer) are ok with it then we should just do this. 
> -Filip
>> On Feb 24, 2017, at 20:50, Mark Lam <mark.lam at apple.com> wrote:
>> The JSC VM has this method setExclusiveThread().  Some details:
>> 1. setExclusiveThread() is only used to forego actually locking/unlocking the underlying lock inside JSLock.
>> 2. setExclusiveThread() is only used by WebCore where we can guarantee that the VM will only ever be used exclusively on one thread.
>> 3. the underlying lock inside JSLock used to be a slow system lock.
>> Now that we have fast locking, I propose that we simplify the JSLock code by removing the concept of the exclusiveThread and always lock/unlock the underlying lock.  This also give us the ability to tryLock the JSLock (something I would like to be able to do for something new I’m working on).
>> Does anyone see a reason why we can’t remove the concept of the exclusiveThread?
>> Thanks.
>> Mark
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