Re: [RFC][PATCH 0/3] VM throttling: avoid blocking occasional writers

[Leroy van Logchem <leroy.vanlogchem@wldelft.nl>]

> I'm sorry to piggy-back this thread.
> 
> Could it be what I'm experiencing in the following bugzilla report:
> http://bugzilla.kernel.org/show_bug.cgi?id=7372
> 
> As I explained in the report, I see this issue only since 2.6.18.
> So if your concern is related to mine, what could have changed between
> 2.6.17 and 2.6.18 related to this?

I don't think it's 2.6.x related, it's been under the sheets from start.

Related to your problem in the 7372 bug:

Pages are kept in memory for re-use, which is fast and fine except for:
1) data without re-use value or even single use
2) applications _do not_ advise the kernel how to cache pages related
   to there self generated i/o. POSIX does provide mechanisms to
   properly do so. But the kernel should help these poor apps.

To minimize your MySQL backup cp(1) problem, try this workaround:

cat ./cp_direct.sh 
#!/bin/sh
dd if=$1 of=$2 bs=1M iflag=direct oflag=direct

Combine this with [/etc/sysctl.conf]:

vm.vfs_cache_pressure = 1
vm.dirty_ratio = 2
vm.dirty_background_ratio = 1

This should reduce both the stress on the vm and response latency during
interactive work.

--
Leroy