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SubjectRe: [PATCH V2] block: make nr_requests tunable for loop
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.bi@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.

--
Jens Axboe



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