On Fri, Jan 8, 2016 at 3:31 PM, Linus Torvalds
[off-list ref] wrote:

On Fri, Jan 8, 2016 at 3:15 PM, Andy Lutomirski [off-list ref] wrote:

quoted

Please play around and suggest (and run?) good benchmarks.  It seems
to save around 100ns on cross-process context switches for me.

Interesting. There was reportedly (I never saw it) a test-patch to use
pcids inside of Intel a couple of years ago, and it never got outside
because it didn't make a difference.

I have a copy of that patch, and my code works very differently.  I
only use 3 bits of PCID in this series.  I could probably reduce that
to 2 with little loss.  4 or more would be a waste.

Either things have changed (newer hardware with more pcids perhaps?)
or you did a better job at it.

On my Skylake laptop, all of the PCID bits appear to have at least
some effect.  Whether this means it gets hashed or whether this means
that all of the bits are real, I don't know.  I'll fiddle with it on
an older machine.

--Andy

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