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

[Davidlohr Bueso]

On Mon, 2014-07-21 at 14:31 -0700, Andy Lutomirski wrote:
> On Mon, Jul 21, 2014 at 2:27 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > 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.

fwiw here's what lmbench's lat_ctx says on my system . For 'accuracy', I
kept the runs short.

http://www.stgolabs.net/lat_ctx.png

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