Subject: Re: processes consuming all CPU during I/O on LFS
To: Blair Sadewitz <firstname.lastname@example.org>
From: Thor Lancelot Simon <email@example.com>
Date: 11/16/2006 15:58:57
On Thu, Nov 16, 2006 at 03:28:29PM -0500, Blair Sadewitz wrote:
> I've noticed that if I untar a lot of files on an LFS filesystem,
> toward the very end of the untar (when all files in the archive are
> listed with -v but before it finishes), tar consumes 95%+ CPU for a
> few minutes and things get VERY slow. This does not happen on FFS
> filesystems. Is this just the nature of LFS, a bug, or do I have
> something set up incorrectly? I got my lfs bsize from the -A option
It's a problem with the code that flushes pending writes. Your system
has a lot of free RAM, I expect.
If you look at the I/O statistics for that disk using "systat vmstat"
or iostat while the system is being slow, how many megs/sec is it
writing? You could try this:
1) Take a rough estimate of your disk's write bandwidth in bytes/sec.
Let's say it's 30000000.
2) Divide this by 4096 (on i386) to get pages, 7324 in this case.
3) Divide this by 4, a fudge factor chosen by means I can't explain
well in a short email message, yielding 1831.
4) Check that 1831 * 4096 is larger than your segment size. Since it's
about 7.5MB, you're safe.
5) sysctl -w vfs.lfs.pagetrip=1831
Let me know what the results look like.
Thor Lancelot Simon firstname.lastname@example.org
"The liberties...lose much of their value whenever those who have greater
private means are permitted to use their advantages to control the course
of public debate." -John Rawls