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SubjectRe: [Patch] shm bug introduced with pagecache in 2.3.11
Alan Cox wrote:
>
> > That said, I don't think this can/will be fixed for a 2.4 timeframe,
> > especially as I haven't heard of any real-life usage where it would be an
> > issue..
>
> News servers like Typhoon , high performance threaded web servers (eg Zeus)
>

Do you know if they are using mmap?

>
> Fortunately these guys tend to be using pretty serious I/O subsystems not
> M/O disks and they are fine with 2.2.
>

I did a second test with a faster disk (SCSI-2-narrow 4.5 GB seagate),
and the results were nearly identical: the mmap semaphore kill's around
33% performance if I compare 64 threads with 64 processes. (33% slower
or 50% faster, depending on your point of view)
Please note that the test is extremely I/O bound, ie I defeat read-ahead
with a RNG, and I only read one byte in every page, and the file is far
larger than available memory.

I'll try to find a faster drive (I had somewhere an old 10kRPM wide
SCSI drive), but I would be surprised if the performance drop would be <
30%.

--
Manfred


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