Re: 2.4.23aa2 (bugfixes and important VM improvements for the high end)

[Andrea Arcangeli]

On Wed, Mar 03, 2004 at 08:07:04PM -0800, Andrew Morton wrote:
> That's a larger difference than I expected.  But then, everyone has been

mysql is threaded (it's not using processes that force tlb flushes at
every context switch), so the only time a tlb flush ever happens is when
a syscall or an irq or a page fault happens with 4:4. Not tlb flush
would ever happen with 3:1 in the whole workload (yeah, some background
tlb flushing happens anyways when you type char on bash or move the
mouse of course but it's very low frequency)

(to be fair, because it's threaded it means they also find 512m of
address space lost more problematic than the db using processes, though
besides the reduced address space there would be no measurable slowdown
with 2.5:1.5)

Also the 4:4 pretty much depends on the vgettimeofday to be backported
from the x86-64 tree and an userspace to use it, so the test may be
repeated with vgettimeofday, though it's very possible mysql isn't using
that much gettimeofday as other databases, especially the I/O bound
workload shouldn't matter that much with gettimeofday.

another reason could be the xeon bit, all numbers I've seen were on p3,
that's why I was asking about xeon and p4 or more recent.

all random ideas, just guessing.

> mysteriously quiet with the 4g/4g benchmarking.

indeed.

> A kernel profile would be interesting.  As would an optimisation effort,
> which, as far as I know, has never been undertaken.

yes, though I doubt you'll find anything interesting in the kernel, the
slowdown should happen because the userspace runs slower, it's like
undercloking the cpu, it's not a bottleneck in the kernel that can be
optimized (at least unless there are bugs in the patch which I think not).
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