linux-kernel - Re: [PATCH V2] block: make nr

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Date:	Mon, 09 Jun 2014 21:12:15 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Junxiao Bi <junxiao.bi@...cle.com>
CC:	andi@...as.de, linux-kernel@...r.kernel.org
Subject: Re: [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@...x mnt]# cat /sys/block/loop0/queue/nr_requests
>>> 128
>>> [root@...x 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@...x mnt]#
>>> [root@...x mnt]# echo 1024 > /sys/block/loop0/queue/nr_requests
>>> [root@...x mnt]# cat /sys/block/loop0/queue/nr_requests
>>> 1024
>>> [root@...x 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@...cle.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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