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SubjectRe: [PATCH] rq-dyn-alloc, dynamic request allocation
Jens Axboe <axboe@suse.de> wrote:
>
> Hi,
>
> This patch adds dynamic request allocation to the block io path. On
> systems with lots of disks (and thus queues) it saves a non-significant
> amount of low memory. It also allows for much better experimentation
> with larger queue lengths, this experimental patch tops the queue depth
> off at 16384 (vs 128 before).

heh, 16k requests per queue? Last time I played with 1024 certain popular
benchmarks ran like a bullet.

> Please play with it. Andrew, want a version for -mm?

Would be much appreciated, thanks.

> */
> static struct request *get_request_wait(request_queue_t *q, int rw)
> {
> - DEFINE_WAIT(wait);
> - struct request_list *rl = &q->rq[rw];
> struct request *rq;
>
> - spin_lock_prefetch(q->queue_lock);
> -
> generic_unplug_device(q);
> do {
> - int block = 0;
> + rq = get_request(q, rw, GFP_NOIO);
>
> - prepare_to_wait_exclusive(&rl->wait, &wait,
> - TASK_UNINTERRUPTIBLE);
> - spin_lock_irq(q->queue_lock);
> - if (!rl->count)
> - block = 1;
> - spin_unlock_irq(q->queue_lock);
> -
> - if (block)
> + if (!rq)
> io_schedule();

hmm. I fear that if a SCHED_FIFO/SCHED_RR task hits this, it will just pick
itself to run again in the schedule() and the box locks up.

A blk_congestion_wait(WRITE, HZ/50) may be better here. It will send the
caller to sleep until someone puts a write request back, which seems
appropriate.


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