BFQ: simple elevator

[Raymond Jennings]

On Sat, Mar 23, 2013 at 9:42 AM, Matthias Brugger
<matthias.bgg at gmail.com> wrote:
> On 03/23/2013 01:05 AM, Raymond Jennings wrote:
>
> On Fri, Mar 22, 2013 at 2:20 PM,  <Valdis.Kletnieks at vt.edu> wrote:
>
> On Fri, 22 Mar 2013 13:53:45 -0700, Raymond Jennings said:
>
> The first heap would be synchronous requests such as reads and syncs
> that someone in userspace is blocking on.
>
> The second is background I/O like writeback and readahead.
>
> The same distinction that CFQ completely makes.
>
> Again, this may or may not be a win, depending on the exact workload.
>
> If you are about to block on a userspace read, it may make sense to go ahead
> and tack a readahead on the request "for free" - at 100MB/sec transfer and
> 10ms
> seeks, reading 1M costs the same as a seek.  If you read 2M ahead and save 3
> seeks later, you're willing.  Of course, the *real* problem here is that how
> much readahead to actually do needs help from the VFS and filesystem levels
> -
> if there's only 600K more data before the end of the current file extent,
> doing
> more than 600K of read-ahead is a loss.
>
> Meanwhile, over on the write side of the fence, unless a program is
> specifically using O_DIRECT, userspace writes will get dropped into the
> cache
> and become writeback requests later on.  So the vast majority of writes will
> usually be writebacks rather than syncronous writes.
>
> So in many cases, it's unclear how much performance CFQ gets from making
> the distinction (and I'm positive that given a sufficient supply of pizza
> and
> caffeine, I could cook up a realistic scenario where CFQ's behavior makes
> things worse)...
>
> Did I mention this stuff is tricky? :)
>
> Oh I'm well aware that it's tricky.  but as I said i'm more interested
> in learning the api than tuning performance.
>
> Having a super efficient toaster won't do much good if I can't plug
> the darn thing in.
>
>
> If you want to understand the interface, I would recommend to start having a
> look to the noop scheduler. It's by far the simplest implementation of a
> scheduler.
>
> For me a good starting point were this slides:
> http://www.cs.ccu.edu.tw/~lhr89/linux-kernel/Linux%20IO%20Schedulers.pdf
>
> Hope that helps you to bring the theory into practice :)
>
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>

Just what I was looking for.

Now, how do I enable/disable my scheduler during kernel config?