Re: Serious Kernel slowdown with HIMEM (4Gig) in 2.6.7

[William Lee Irwin III <wli@holomorphy.com>]

On Fri, Aug 13, 2004 at 10:11:23PM -0400, Lawrence E. Freil wrote:
> I'm following the kernel bug reporting format so:
> 1. Linux 2.6.7 kernel slowdown in directory access with HIMEM on
> 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.

Please try to reproduce this with CONFIG_HIGHMEM=y but using mem=700M
This will tell me something useful beyond "boot with less RAM".

-- wli