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SubjectRe: Serious Kernel slowdown with HIMEM (4Gig) in 2.6.7
> 2. I have discovered an issue with the Linux 2.6.7 kernel when HIMEM is
> enabled which exhibits itself as a slowdown in directory access regardless
> of filesystem used. When HIMEM is disabled the performance returns to
> normal. The test I ran was a simple "/usr/bin/time ls -l" of a directory
> with 3000 empty files. With HIMEM enabled in the kernel this takes
> approximately 1.5 seconds. Without HIMEM it takes 0.03 seconds. The
> time is 100% CPU and no I/O operations are done to disk. "time" reports
> there are 460 "minor" page faults with zero "major" page faults.
> I believe the issue here is the mapping of pages between high-mem and
> lowmem in the kernel paging code. This increase in time for directory
> accesses doubles to triples times for applications using samba.
> I have also tested this on another system which had only 512Meg of RAM
> but with HIMEM set in the kernel and did not experience the problem.
> I believe it only effects the performance when the paging buffers end
> up in highmem.

Does time indicate more systime or user time? If it's systime (probably is)
please take a kernel profile of each (see Documentation/basic_profiling.txt).
That'll give us a much better clue what's going on.

M.

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