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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