It does not seem to be possible to reproduce the error with apache2. But
this may be because Apache2's i/o handling doesn't impress much. With Tux,
I keep getting up to 40 megs per sec, but with Apache the average is
~15MB/s.
Btw ... It looks like your patch (against rmap12a) gave me an extra
performance kick. 12c gave me a max of ~32MB/s, whereas your patch
highered this to ~41.
thanks
roy
On Thu, 31 Jan 2002, Jens Axboe wrote:
> On Thu, Jan 31 2002, Andrew Morton wrote:
> > rmap 11c:
> > ...
> > - elevator improvement (Andrew Morton)
> >
> > Which includes:
> >
> > - queue_nr_requests = 64;
> > - if (total_ram > MB(32))
> > - queue_nr_requests = 128; + queue_nr_requests = (total_ram >> 9) & ~15; /* One per half-megabyte */
> > + if (queue_nr_requests < 32)
> > + queue_nr_requests = 32;
> > + if (queue_nr_requests > 1024)
> > + queue_nr_requests = 1024;
> >
> >
> > So Roy is running with 1024 requests.
>
> Ah yes, of course.
>
> > The question is (sorry, Roy): does this need fixing?
> >
> > The only thing which can trigger it is when we have
> > zillions of threads doing reads (or zillions of outstanding
> > aio read requests) or when there are a large number of
> > unmerged write requests in the elevator. It's a rare
> > case.
>
> Indeed.
>
> > If we _do_ need a fix, then perhaps we should just stop
> > using READA in the readhead code? readahead is absolutely
> > vital to throughput, and best-effort request allocation
> > just isn't good enough.
>
> Hmm well. Maybe just a small pool of requests set aside for READA would
> be a better idea. That way "normal" reads are not able to starve READA
> completely.
>
> Something ala this, completely untested. Will try and boot it now :-)
> Roy, could you please test? It's against 2.4.18-pre7, I'll boot it now
> as well...
>
> --- /opt/kernel/linux-2.4.18-pre7/include/linux/blkdev.h Mon Nov 26 14:29:17 2001
> +++ linux/include/linux/blkdev.h Thu Jan 31 22:29:01 2002
> @@ -74,9 +74,9 @@
> struct request_queue
> {
> /*
> - * the queue request freelist, one for reads and one for writes
> + * the queue request freelist, one for READ, WRITE, and READA
> */
> - struct request_list rq[2];
> + struct request_list rq[3];
>
> /*
> * Together with queue_head for cacheline sharing
> --- /opt/kernel/linux-2.4.18-pre7/drivers/block/ll_rw_blk.c Sun Jan 27 16:06:31 2002
> +++ linux/drivers/block/ll_rw_blk.c Thu Jan 31 22:36:24 2002
> @@ -333,8 +333,10 @@
>
> INIT_LIST_HEAD(&q->rq[READ].free);
> INIT_LIST_HEAD(&q->rq[WRITE].free);
> + INIT_LIST_HEAD(&q->rq[READA].free);
> q->rq[READ].count = 0;
> q->rq[WRITE].count = 0;
> + q->rq[READA].count = 0;
>
> /*
> * Divide requests in half between read and write
> @@ -352,6 +354,20 @@
> q->rq[i&1].count++;
> }
>
> + for (i = 0; i < queue_nr_requests / 4; i++) {
> + rq = kmem_cache_alloc(request_cachep, SLAB_KERNEL);
> + /*
> + * hey well, this needs better checking (as well as the above)
> + */
> + if (!rq)
> + break;
> +
> + memset(rq, 0, sizeof(struct request));
> + rq->rq_status = RQ_INACTIVE;
> + list_add(&rq->queue, &q->rq[READA].free);
> + q->rq[READA].count++;
> + }
> +
> init_waitqueue_head(&q->wait_for_request);
> spin_lock_init(&q->queue_lock);
> }
> @@ -752,12 +768,18 @@
> req = freereq;
> freereq = NULL;
> } else if ((req = get_request(q, rw)) == NULL) {
> - spin_unlock_irq(&io_request_lock);
> +
> if (rw_ahead)
> - goto end_io;
> + req = get_request(q, READA);
>
> - freereq = __get_request_wait(q, rw);
> - goto again;
> + spin_unlock_irq(&io_request_lock);
> +
> + if (!req && rw_ahead)
> + goto end_io;
> + else if (!req) {
> + freereq = __get_request_wait(q, rw);
> + goto again;
> + }
> }
>
> /* fill up the request-info, and add it to the queue */
> @@ -1119,7 +1141,7 @@
> */
> queue_nr_requests = 64;
> if (total_ram > MB(32))
> - queue_nr_requests = 128;
> + queue_nr_requests = 256;
>
> /*
> * Batch frees according to queue length
>
> --
> Jens Axboe
>
-- Roy Sigurd Karlsbakk, MCSE, MCNE, CLS, LCAComputers are like air conditioners. They stop working when you open Windows.
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This archive was generated by hypermail 2b29 : Thu Feb 07 2002 - 21:00:15 EST