Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

On Mon, 21 Jul 2014, Andy Lutomirski wrote:

On Mon, Jul 21, 2014 at 2:27 PM, Peter Zijlstra [off-list ref] wrote:

quoted

All this is predicated on the fact that syscalls are 'expensive'.
Weren't syscalls only 100s of cycles? All this bitmap mucking is far
more expensive due to cacheline misses, which due to the size of the
things is almost guaranteed.

120 - 300 cycles for me, unless tracing happens, and I'm working on
reducing the incidence of tracing.

So it's a non issue indeed and definitely not worth the trouble of
that extra storage, the scheduler overhead, etc.

Summary: Once you can't take it atomically in user space, you've lost
	 anyway. And we are better off to do the magic spinning in
	 kernel where we have all the information accessible already.

Thanks,

	tglx