cmp(1) has a bottleneck, but where?
dieterbsd at engineer.com
Thu Jan 5 17:46:28 UTC 2012
> Something/somehow it's issuing smaller IOs when using mmap?
On my box, 64K reads. Using the '-' to avoid mmap it uses
The big difference I found was that the default mmap case isn't
using read-ahead. So it has to wait on the disk every time. :-(
Using the '-' to avoid mmap it benefits from read-ahead, but the
default of 8 isn't large enough. Crank up vfs.read_max and it
becomes cpu bound. (assuming using 2 disks and not limited by
both disks being on the same wimpy controller)
A) Should the default vfs.read_max be increased?
B) Can the mmap case be fixed? What is the aledged benefit of
using mmap anyway? All I've even seen are problems.
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