Rik van Riel wrote:
Nick Piggin wrote:
OK, sure. I think we need more numbers though.
Thinking about the issue some more, I think I know just the
number we might want to know.
It is pretty obvious that the kernel needs to do less work
with the MADV_FREE code present. However, it is possible
that userspace needs to do more work, by accessing pages
that are not in the CPU cache, or in another CPU's cache.
In the test cases where you see similar performance on the
workload with and without the MADV_FREE code, are you by any
chance seeing lower system time and higher user time?
I didn't actually check system and user times for the mysql
benchmark, but that's exactly what I had in mind when I
mentioned the poor cache behaviour this patch could cause. I
definitely did see user times go up in benchmarks where I
measured.
We have percpu and cache affine page allocators, so when
userspace just frees a page, it is likely to be cache hot, so
we want to free it up so it can be reused by this CPU ASAP.
Likewise, when we newly allocate a page, we want it to be one
that is cache hot on this CPU.
I think that maybe for 2.6.22 we should just alias MADV_FREE
to run with the MADV_DONTNEED functionality, so that the glibc
people can make the change on their side while we figure out
what will be the best thing to do on the kernel side.
I'll send in a patch that does that once Linus has committed
your most recent flood of patches. What do you think?
I'll let you and Ulrich decide on that. Keep in mind that older
kernels (without the mmap_sem patch for MADV_DONTNEED) still
seem to get a pretty decent improvement from using MADV_DONTNEED,
so it is possible glibc will want to start using that anyway.
--
SUSE Labs, Novell Inc.
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