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Subject2.4.17: pwrite destroys block I/O throughput
From
Date
Hi there,

I've never heard of pwrite and pread before, but htdig apparently makes
very heavy use of it. On my 2.4.17 SMP 2GB HIGHMEM + ext3 system,
running htdig destroys input rates for every other process. htdig's
output proceeds at approximately 2MB/s, but input for the entire system
runs only at about 4KB/s (YES, KB!). If I suspend htdig, system block
input increases to the normal rate of 10-50MB/s. Output still works, as
a dd from /dev/zero to a 400MB file runs at about 25MB/s.

Is linux's pwrite() just horribly broken? Is htdig the only program
that uses it?

Here's a little snapshot of htdig's syscalls, strace -s 0:

pwrite(6, ""..., 8192, 20717568) = 8192
pread(6, ""..., 8192, 138395648) = 8192
pwrite(6, ""..., 8192, 127918080) = 8192
pread(6, ""..., 8192, 179281920) = 8192
pwrite(6, ""..., 8192, 79732736) = 8192
pread(6, ""..., 8192, 137633792) = 8192
pwrite(6, ""..., 8192, 28409856) = 8192
pread(6, ""..., 8192, 157958144) = 8192
pwrite(6, ""..., 8192, 96583680) = 8192
pread(6, ""..., 8192, 141254656) = 8192
pwrite(6, ""..., 8192, 131031040) = 8192
pread(6, ""..., 8192, 19095552) = 8192
pwrite(6, ""..., 8192, 82698240) = 8192
pread(6, ""..., 8192, 170573824) = 8192
pwrite(6, ""..., 8192, 152616960) = 8192
pread(6, ""..., 8192, 207298560) = 8192
pwrite(6, ""..., 8192, 148635648) = 8192
pread(6, ""..., 8192, 202768384) = 8192
pwrite(6, ""..., 8192, 174055424) = 8192

It's seeking all over the place. Maybe pwrite/pread bypass the elevator
and proper I/O scheduling.

-jwb

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