Re: [PATCH] livelock in elevator scheduling

[Jens Axboe]

On Tue, Nov 21 2000, [e-mail] wrote:
> The current elevator_linus() doesn't obey the true elevator
> scheduling, and causes I/O livelock during frequent random write
> traffics. In such environment I/O (read/write) transactions may be
> delayed almost infinitely (more than 1 hour).
> 
> Problem:
>  Current elevator_linus() traverses the I/O requesting queue from the
> tail to top. And when the current request has smaller sector number
> than the request on the top of queue, it is always placed just after
> the top.
>  This means, if requests in some sector range are continuously
> generated, a request with larger sector number is always places at the
> last and has no chance to go to the front.  e.g. it is not scheduled.

Believe it or not, but this is intentional. In that regard, the
function name is a misnomer -- call it i/o scheduler instead :-)
The current settings in test11 cause this behaviour, because the
starting request sequence numbers are a 'bit' too high.

I'd be very interested if you could repeat your test with my
block patch applied. It has, among other things, a more fair (and
faster) insertion.

*.kernel.org/pub/linux/kernel/people/axboe/patches/2.4.0-test11/blk-11.bz2

> [...] Additionally, it may be better to add extra priority to reads
> than writes to obtain better response, but this patch doesn't.

READs do have bigger priority, they start out with lower sequence
numbers than WRITEs do:

	latency = elevator_request_latency(elevator, rw);

With my patch, READ sequence start is now 8192. WRITEs are twice
that.

-- 
* Jens Axboe <[e-mail]>
* SuSE Labs