default i/o scheduler changed? [Solved]

Bones McCracker · Veteran Joined: 14 Mar 2006 Posts: 1611 Location: U.S.A.

Upgrading from v2.6.20-gentoo-r8 to v2.6.21-gentoo-r4, I noticed a change to the comments in the kernel config that indicate CFQ is now the default I/O scheduler (although anticipatory was still set as the default). I'm looking for some insight into this change.

2.6.20-r8 indicates that Anticipatory is the default I/O scheduler:

didymos · Posted: Mon Jul 16, 2007 9:02 am Post subject:

Some forgot to fix the defconfig bit. It's set correctly when doing "make defconfig" with 2.6.22.
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Thomas S. Howard

Bones McCracker · Veteran Joined: 14 Mar 2006 Posts: 1611 Location: U.S.A.

Thanks.

I'm still curious why the change.

broken_chaos · Posted: Mon Jul 16, 2007 7:28 pm Post subject:

CFQ is better, to put it simply. Much, much, much better in general for desktop/laptop/workstation I/O speeds and fairness between processes. Anticipatory or Deadline can be better for certain servers, but server admins should know what they're doing much more often than desktop/laptop users.

Bones McCracker · Veteran Joined: 14 Mar 2006 Posts: 1611 Location: U.S.A.

I guess things have changed then. People used to say CFQ was good for things you didn't want hiccups on (like video, etc.) but that anticipatory got much better performance in general. Sounds like there have been improvements to the CFQ scheduler.

Thanks for the insight.

broken_chaos · Posted: Mon Jul 16, 2007 9:25 pm Post subject:

Just for a little more, from the kernel configuration:
"The CFQ I/O scheduler tries to distribute bandwidth equally among all processes in the system. It should provide a fair working environment, suitable for desktop systems."

It tries to be fair to all processes, which is more important on a desktop than one process having all the disk I/O - 90% of the time you'll not just have *one* program *only* doing disk access - your IM client might be writing a logfile, or reading a configuration file, while your browser accesses/saves cache files, and your cron daemon is doing a updatedb for locate, for instance. All these don't happen at the same time, but I believe they're supposed to be 'sharing' the I/O much better - for instance if you throw a copy of a large file into there, the other processes will still get disk I/O in a reasonable time.

(Well, I think that's the theory - it's obviously not *perfect* yet, but it's, on average, better for desktops/laptops than the other two currently.)