Re: [PATCH] cfq-prio #2

[Nick Piggin <piggin@cyberone.com.au>]

Jens Axboe wrote:

>On Tue, Nov 11 2003, Nick Piggin wrote:
>
>>
>>Jens Axboe wrote:
>>
>>
>>>Hi,
>>>
>>>
>>>
>>Hi Jens
>>
>>
>>>@@ -1553,6 +1559,10 @@
>>>	struct io_context *ioc = get_io_context(gfp_mask);
>>>
>>>	spin_lock_irq(q->queue_lock);
>>>+
>>>+	if (!elv_may_queue(q, rw))
>>>+		goto out_lock;
>>>+
>>>	if (rl->count[rw]+1 >= q->nr_requests) {
>>>		/*
>>>		 * The queue will fill after this allocation, so set it as
>>>@@ -1566,15 +1576,12 @@
>>>		}
>>>	}
>>>
>>>-	if (blk_queue_full(q, rw)
>>>-			&& !ioc_batching(ioc) && !elv_may_queue(q, rw)) {
>>>
>>>
>>I know I hijacked elv_may_queue from you... any chance we could seperate
>>these so our schedulers can live in peace? ;)
>>
>
>IOW, you completely broke it! I'm just changing it back to the
>original. When was this done, btw? Just discovered it when updating the
>patch. Pretty annoying...
>

You acked the change actually :P
I guess it was done in mainline when AS was merged.

>
>>Maybe my version should be called elv_force_queue?
>>
>
>I just hate to see more of these, really. The original idea for
>may_queue was just that, may this process queue io or not. We can make
>it return something else, though, to indicate whether the process must
>be able to queue. Is it really needed?
>

Its quite important. If the queue is full, and AS is waiting for a process
to submit a request, its got a long wait.

Maybe a lower limit for per process nr_requests. Ie. you may queue if this
queue has less than 128 requests _or_ you have less than 8 requests
outstanding. This would solve my problem. It would also give you a much more
appropriate scaling for server workloads, I think. Still, thats quite a
change in behaviour (simple to code though).