On 06/10/2014 11:12 AM, Jens Axboe wrote: > On 2014-06-09 20:50, Junxiao Bi wrote: >> On 06/10/2014 10:41 AM, Jens Axboe wrote: >>> On 2014-06-09 20:31, Junxiao Bi wrote: >>>> commit 7b5a3522 (loop: Limit the number of requests in the bio list) >>>> limit >>>> the request number in loop queue to not over 128. Since the >>>> "request_fn" of >>>> loop device is null, the requests number is not allowed tuned. Make >>>> it tunable >>>> from sysfs can improve performance. >>>> >>>> The following test is done on a machine with 512M memory. The >>>> backend of >>>> /dev/loop1 is a nfs file. >>>> >>>> [root@bijx mnt]# cat /sys/block/loop0/queue/nr_requests >>>> 128 >>>> [root@bijx mnt]# dd if=/dev/zero of=/dev/loop0 bs=1M count=5000 >>>> 5000+0 records in >>>> 5000+0 records out >>>> 5242880000 bytes (5.2 GB) copied, 501.572 s, 10.5 MB/s >>>> [root@bijx mnt]# >>>> [root@bijx mnt]# echo 1024 > /sys/block/loop0/queue/nr_requests >>>> [root@bijx mnt]# cat /sys/block/loop0/queue/nr_requests >>>> 1024 >>>> [root@bijx mnt]# dd if=/dev/zero of=/dev/loop0 bs=1M count=5000 >>>> 5000+0 records in >>>> 5000+0 records out >>>> 5242880000 bytes (5.2 GB) copied, 464.481 s, 11.3 MB/s >>>> >>>> Signed-off-by: Junxiao Bi <junxiao...@oracle.com> >>>> --- >>>> block/blk-core.c | 6 ++++++ >>>> block/blk-sysfs.c | 9 +++------ >>>> 2 files changed, 9 insertions(+), 6 deletions(-) >>>> >>>> diff --git a/block/blk-core.c b/block/blk-core.c >>>> index 40d6548..58c4bd4 100644 >>>> --- a/block/blk-core.c >>>> +++ b/block/blk-core.c >>>> @@ -851,6 +851,12 @@ int blk_update_nr_requests(struct request_queue >>>> *q, unsigned int nr) >>>> q->nr_requests = nr; >>>> blk_queue_congestion_threshold(q); >>>> >>>> + /* for loop device, return after set its nr_requests */ >>>> + if (!q->request_fn) { >>>> + spin_unlock_irq(q->queue_lock); >>>> + return 0; >>>> + } >>> >>> It'd be prettier to split this differently - something ala: >>> >>> if (request_fn) >>> blk_update_congestion_thresholds(q); >> The congestion threshholds is needed in commit 7b5a3522 (loop: Limit the >> number of requests in the bio list). So I think it needs be set even >> request_fn is null. > > I mean the request list thresholds, the part below where you currently > just exit. > >>> But I think you have a larger issue here... For the request lists, we >>> update the congestion thresholds and wakeup anyone waiting, if we need >>> to. There's no way to do that for loop, since the waitqueue is >>> internal to loop. >> Loop do the congestion control by itself, in loop_make_request() / >> loop_thread(). > > Yes, that is my point! You update nr_congestion_off, but you don't > wake anyone currently sitting in wait_event_lock_irq() on that value. > See what the code below where you just exit does for request list > based devices. Ah, i see. It can't be wake up once nr_congestion_off is updated. But after a little delay, loop_thread will consume the requests in list and wake up it. Is this OK?
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