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From "Aleksey Shipilev" <aleksey.shipi...@gmail.com>
Subject [drlvm][startup][performance] Implementing futex'es
Date Tue, 12 Feb 2008 23:12:13 GMT
Hi, Alexei, all,

Just another idea for startup optimizations pops out of our talk with
Egor Pasko. :)

As you probably know there are many places in VM and JIT that use
locking for safety reasons. Most of this locking is driven by mutexes,
that is, the kernel calls. That's a good option in case of contention,
because such locking will need arbitration (e.g. "who will take the
mutex next"?) from kernel side. But what if that locking is not
contended? Even then we will make the kernel call for trying to catch
the mutex.

Linux has long ago implemented such thing as "fast user-space mutex",
"futex" [1]. Generally it is simple memory region that could be
incremented/decremented atomically. In case of contention futex, of
course, will resort to kernel-side mutex.

That mean we could save precious time using futexes instead of
mutexes: we definitely will save on kernel call time.

AFAIK, current implementation of porting layer has no support for
futexes even on Linux side. And so we might try to implement them for
Windows part and use the Linux-provided futex'es on Linux part. Then
after the implementation of hyfutex_lock/unlock we might try to
migrate performance-significant locks to futexes one-by-one. Profilers
are good directions, maybe anywhere else too.

What do you think?

Thanks,
Aleksey,
ESSD, Intel

[1] http://en.wikipedia.org/wiki/Futex

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