Re: [PATCH -mm] [3/3] Add the Elevator I/O scheduler

From: Nate Diller
Date: Fri Aug 04 2006 - 14:15:09 EST

On 8/4/06, Al Boldi <a1426z@xxxxxxxxx> wrote:
Nate Diller wrote:
> This is the Elevator I/O scheduler. It is a simple one-way elevator,

Thanks for yet another attempt to achieve an efficient 2.6 elevator.

hopefully it's useful for you, I'd love to see performance results one
way or the other.

> +static inline char contig_char(struct el_request *e)

You probably meant el_req. Check the rest.

> +static void print_queue(struct request_queue *q, struct el_data *el)
> +{
> + struct el_req *e;
> +
> + printel(el);

Should be print_el_data.

Applied against 2.6.17, it boots with this:
io scheduler elevator registered (default)
elevator: forced dispatching is broken (nr_sorted=13), please report this

damn, i thought i fixed that.

> +In pure form its largest weakness is starvation of other processes due to
> +one process writing a very large number of contiguous requests (e.g.
> +tarring a very large tar file while other processes are trying to run).

cat /dev/hda > /dev/null starves the rest of the system.

yeah, that can be worse on some systems than others, it seems to vary
a bit with file system and device driver. some setups cause more
"breaks" in the stream of requests.

> +The max_contig and max_write tunables are two (imperfect) solutions. They
> +
> +These two tunables are still under construction, but they have proven
> +somewhat useful in practice. Usually, max_contig should be the same size
> +(in bytes) as ra_pages.

It's 0 by default. Setting it to ra_pages prints this:
nate 1977: max_contig went backwards

hmmm, haven't tested this recently. dunno why that would have broken...

> +
> + SSTF
> +
> +The SSTF feature was added on a whim. It ignores the tunables, and
> probably +breaks tracing. I didn't ever see it perform better than SCAN,
> but who +knows?

Starves too.

oh, yeah, definitely, this algorithm is only suitable for a workload
where you want latency over fairness. suppose i should mention that.

Thanks for your comments and testing

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