Re: Cache queue_congestion_on/off_threshold

[Jens Axboe]

On Thu, May 06 2004, Chen, Kenneth W wrote:
> >>>> Andrew Morton wrote on Wednesday, May 05, 2004 11:34 PM
> > Jens Axboe <axboe@xxxxxxx> wrote:
> > >
> > > Do you have any numbers at all for this? I'd say these calculations are
> > >  severly into the noise area when submitting io.
> >
> > The difference will not be measurable, but I think the patch makes sense
> > regardless of what the numbers say.
> 
> Even though it is in the noise range that can't be easily measured, they are
> indeed in the positive territory.  If I stack 5 of these little things, we
> actually measured positive gain on a large db workload.

I somehow still find that very hard to believe, it's a branch and a
couple of cycles.

> There isn't anything absurd in 2.6 kernel, however, I hate to say that we
> consistently see performance regression with latest 2.6 kernel compare to
> best 2.4 based kernel under heavy db workload on 4-way SMP platform. (2.6
> rocks on numa platform that 2.4 doesn't even have a chance to compete).
> 
> Some of the examples are:
> 
> (1) it's cheaper to find out whether a queue is empty or not by calling
>     elv_queue_empty() instead of using heavier elv_next_request().
> (2) it's better to check queue empty before calling into q->request_fn()
> 
> 
> diff -Nurp linux-2.6.6-rc3/drivers/block/ll_rw_blk.c linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c
> --- linux-2.6.6-rc3/drivers/block/ll_rw_blk.c	2004-05-06 13:03:14.000000000 -0700
> +++ linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c	2004-05-06 13:04:04.000000000 -0700
> @@ -1128,7 +1128,7 @@ static inline void __generic_unplug_devi
>  	/*
>  	 * was plugged, fire request_fn if queue has stuff to do
>  	 */
> -	if (elv_next_request(q))
> +	if (!elv_queue_empty(q))
>  		q->request_fn(q);
>  }
> 
> @@ -1237,7 +1237,8 @@ void blk_run_queue(struct request_queue
> 
>  	spin_lock_irqsave(q->queue_lock, flags);
>  	blk_remove_plug(q);
> -	q->request_fn(q);
> +	if (!elv_queue_empty(q))
> +		q->request_fn(q);
>  	spin_unlock_irqrestore(q->queue_lock, flags);
>  }

This looks great, should be merged right away.

> (3) can we allocate request structure up front in __make_request?
>     For I/O that cannot be merged, the elevator code executes twice
>     in __make_request.
> 
> 
> diff -Nurp linux-2.6.6-rc3/drivers/block/ll_rw_blk.c linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c
> --- linux-2.6.6-rc3/drivers/block/ll_rw_blk.c	2004-05-06 13:03:14.000000000 -0700
> +++ linux-2.6.6-rc3.ken/drivers/block/ll_rw_blk.c	2004-05-06 13:11:39.000000000 -0700
> @@ -2154,15 +2154,14 @@ static int __make_request(request_queue_
> 
>  	ra = bio->bi_rw & (1 << BIO_RW_AHEAD);
> 
> +	/* Grab a free request from the freelist */
> +	freereq = get_request(q, rw, GFP_ATOMIC);
> +
>  again:
>  	spin_lock_irq(q->queue_lock);
> 
> -	if (elv_queue_empty(q)) {
> +	if (elv_queue_empty(q))
>  		blk_plug_device(q);
> -		goto get_rq;
> -	}
> -	if (barrier)
> -		goto get_rq;
> 
>  	el_ret = elv_merge(q, &req, bio);
>  	switch (el_ret) {

Actually, with the good working batching we might get away with killing
freereq completely. Have you tested that (if not, could you?)

> Some more, I will post in another thread.

Can you please remember to cc in initial posts as well, I don't want to
always hunt for your findings. Thanks.

-- 
Jens Axboe

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