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

On Mon, 2014-07-21 at 14:31 -0700, 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.

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