Scheduler questions.

[madbiker]

I searched the forums, but didn't really see much that really explained things well.

So from what I've read, there are I/O and CPU schedulers, and there are a few major choices for each. Different kernels include different schedulers in them, and different ones work better for different purposes.

Using ck sources, I compiled in support for all 3 schedulers that I was given the option for. I can change which one I'm using by passing elevator=deadline or elevator=cfq or whichever one I want in my grub.conf. So these are I/O schedulers correct? Is there any way to determine which one I'm currently using? How about CPU schedulers? How do I know which one I'm using, how do I change?

Finally, are there generally accepted standards for schedulers for desktop use and server use? I imagine some of them are going to be considered "unstable" or in "testing" phases, but while being unstable also provide better desktop performance (much like ck, love and nitro kernel sources)?

Thanks for any light you can shed on the subject.

[choco]

I'm interested in learning more about the various schedulers too. http://kerneltrap.org has been an informative site for this sort of thing, although it's so technical (it refers directly to the email lists of kernel developers).

I'm not as sure about CPU schedulers, but you can tell which I/O scheduler is in use by looking at the kernel boot log. Doing a

[commonloon]

You can find some documentation in /usr/src/linux/Documentation/block where linux is the symlink to your kernel version.

2.6 has the as (default - anticipatory), cfq (completely fair), deadline (good for databases) and noop (good for devices such as usb drives).

You can find out your scheduler via dmesg as described above or in /sys/block/sdb/queue/scheduler on _some_ newer (2.6.10?) or patched kernels. On these kernels you may change it w/o rebooting by echo cfq > /sys/block/sdb/queue/scheduler (sdb is the device in question) for example -- auto-magically you'll then notice it will look like:

porky queue # cat scheduler
noop anticipatory deadline [cfq]

You can adjust io scheduler parameters by adjusting things in: /sys/block/sda/queue/iosched where sda is the device your interested in changing. The parameters and files within the iosched dir vary -- see the respective documentation.... seems more options (and more complex) that elvtune for 2.4. You can change via standard echo value > iosched/param

Hope this helps...
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[madbiker]

Can't believe I missed this reply.

Yes that helps a lot! I wasn't getting any mention of which scheduler I was using with dmesg, but I'm using a 2.6.10 and had that scheduler bit in my /sys. Works great.

Now my only questions relate to CPU schedulers. I'm using ck-sources, so does that mean I'm using the 'staircase' scheduler automagically? Is it simply determined by the kernel patchset you're using, and there's no way to change it other than go to a patchset that's patched with a different scheduler?

[steve_d555]

I have a question, I hear that Deadline/Anticipatory is better for gaming as it allows it to take up 100%. Does anyone have any thoughts? Has anyone done any benchmarks against CFQ or something? I would be really interested to know, because ive been using Nitro sources with CFQ and was wondering what was better.

Thanks,
--Steve

[commonloon]

As far as cpu schedulers, I can't help much as I only know enough to be dangerous. In 2.6 there is O(1) "Ultra-scalable" (damn cool name) scheduler that is the main one (/usr/src/Documentation/sched-design.txt). There is also, I think, the SC or staircase and the PB or priority based; I'm pretty sure that you need to have a specific patchset to have either of the alternative cpu schedulers.

Regarding gaming and schedulers -- haven't seen specific benchmarks.

I'm actually in the middle of doing some benchmarks comparing various schedulers and parameters for a database server. I'll post some results when I'm done, but they may not help w/ destop or gaming.
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[steve_d555]

OK, Ya I heard somewhere that the Staircase/CFQ is not very good for gaming as it doesnt allow users to take 100% of the CPU or something.
I heard that the best setup would be the default Scheduler with Deadline in gentoo sources. I'm not sure, but I might try testing it with a new rig I'm getting just to try and get the full performance outta my 6600GT

--Steve

[oggialli]

Hmm, you probably won't gain anything in games using those things compared to what you gain using cfqv2 and a recent staircase in multitasking/general desktop usage. For example I get 64,1 fps in Doom3 timedemo demo1 usecache second run with 1024x768, 4xAA, 8xAF, Ultra Quality. I think that is enough, and with the stock scheduler my desktop feels a lot more choppy.

Also, batch scheduling absolutely rocks.
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[masseya]

Moving from Gentoo Chat.
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[fallow]

about O(1),Staircase and SPA Family also EBS and Nicksched is in many of threads on this forum , It was been talked many times but casue of many of threads it`s difficult to search every sentence/post ..but possible

maybe someone will be interested in easy io scheduler changing script - I`m using it all the time

https://forums.gentoo.org/viewtopic.php?p=1945476#1945476

and IMHO : https://forums.gentoo.org/viewtopic.php?p=2053237#2053237

also about bonnie++ small comparison on whole reiser4 system beetwen staircase / O(1) / nicksched .
https://forums.gentoo.org/viewtopic.php?p=2076954#2076954
( results : http://vivid.dat.pl/fallow/sched_test/results.html )
cheers
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[commonloon]

...Thanks Fallow for the info....

What follows is some of the benchmarks I've been running on a new server. They are only benchmarks (for fun). I don't really think they show much difference between schedulers (I'm sticking w/ deadline, since this is a database server). I'm also testing using a SQL test script to test closer to the real world. The server's specs are:

Tyan 2882 mobo
16GB ram
2x 250 Opterons
2x10K 73GB RAID1 OS
6x15K ~200GB RAID10 data
gentoo-dev-sources 2.6.9-r14

I have booted w/ mem=4G to make the tests run a bit quicker, as w/ 16GB of ram running a non-caching test (~32GB min) is time consuming. I use ext2 instead of xfs (what I will be using), so that I could eliminate any effects of writing the journal from latency timings. XFS results in better read, write bandwidth and non-write latencies. Also, note that I've found that bonnie++ 1.93c does not show per char K/sec correctly(?), version 1.03 does, and I used 1.93c below.

[commonloon]

This came across LKML. While I haven't read it completely, it seems to have a nice overview of the O(1), etc. scheduler.

http://josh.trancesoftware.com/linux/
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[bollucks]