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SubjectRe: [PFC]: hash instrumentation
On Thu, 8 Apr 1999, Andrea Arcangeli wrote:
> In the meantime only if you have some spare time, could you do a bench
> over my last 2.2.5_arca8.bz2? It has major changes in the buffer code
> (fixed flushtime and fixed the way buffers are flushed to disk by
> bdflush). It works as never before here also under swapping (performances
> visible with eyes ;).

i haven't tried all of arca8 yet, but i did sniff around in the bdflush
logic to see what you tried out. i took a stock 2.2.5 kernel and took out
the "run_task_queue()" in wakeup_bdflush() just to see what that it would
do. that made things a little worse, so i'm guessing you also had some
compensatory changes in other areas.

i'm interested to see how your flushtime changes will effect performance.

> in arca8 I restored the o+offset in the hash function, feel free to remove
> it before running the bench if you think it will harm (the +offset is not
> needed until you'll be very low on memory and it looks like to me that
> your machine never gets low on memory ;).

i'm discovering that a 13 bit hash mitigates the spikey size distribution
in the page hash *better* than the +offset change. although i've been
able to push the system into swap, i still haven't seen any degenerate
hash behavior that's as bad as the buffer cache's hash function.

i'll have more as i test this further.

> Ah and also the first pagemap-cachealigned patch was wrong (blame me). The
> second patch (after Eric pointed out to think about it twice) does the
> right thing (so you may consider to run the bench on it again... excuse
> me). So probably this is the reason that the pagemap-aligned+irq-aligned
> bench was weird...

i suspected that the strange performance behavior wasn't an implementation
problem, so i tried a plain 2.2.5 kernel with just the page struct
alignment mod in your latest patch. the numbers were about the same as
before. it would probably be a good idea if someone could actually take a
close look at what the alignment mod is doing to change hardware behavior,
'cause it's obviously not what we expect.

- Chuck Lever
--
corporate: <chuckl@netscape.com>
personal: <chucklever@netscape.net> or <cel@monkey.org>

The Linux Scalability project:
http://www.citi.umich.edu/projects/citi-netscape/


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