Re: Re: Why blktrace didn't trace requests merge?

["Jianpeng Ma" <majianpeng@gmail.com>]

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On 2012-09-18 13:49 Jens Axboe <axboe@kernel.dk> Wrote:
>On 2012-09-18 02:30, Jianpeng Ma wrote:
>> On 2012-09-18 02:27 Jens Axboe <axboe@kernel.dk> Wrote:
>>> On 2012-09-17 19:55, Tejun Heo wrote:
>>>> (cc'ing Jens)
>>>>
>>>> On Mon, Sep 17, 2012 at 09:22:28AM -0400, Steven Rostedt wrote:
>>>>> On Mon, 2012-09-17 at 19:33 +0800, Jianpeng Ma wrote:
>>>>>> Hi all:
>>>>>> 	I used blktrace to trace some io.But i can't find requests merge. I searched the code and did't not find.
>>>>>> 	Why? 
>>>>>> 									
>>>>>
>>>>> No idea. I don't use blktrace much, but I Cc'd those that understand it
>>>>> better than I.
>>>
>>> Works for me:
>>>
>>> [...]
>>>
>>>
>>>  8,0    0       26     0.009147735   664  A  WS 315226143 + 8 <- (8,7) 19406344
>>>  8,0    0       27     0.009148677   664  Q  WS 315226143 + 8 [btrfs-submit-1]
>>>  8,0    0       28     0.009152967   664  G  WS 315226143 + 8 [btrfs-submit-1]
>>>  8,0    0       29     0.009154242   664  P   N [btrfs-submit-1]
>>>  8,0    0       30     0.009155538   664  A  WS 315226151 + 8 <- (8,7) 19406352
>>>  8,0    0       31     0.009155743   664  Q  WS 315226151 + 8 [btrfs-submit-1]
>>>  8,0    0       32     0.009157086   664  M  WS 315226151 + 8 [btrfs-submit-1]
>>>  8,0    0       33     0.009158716   664  I  WS 315226143 + 16 [btrfs-submit-1]
>>>
>>> That's from a quick trace of /dev/sda. I started blktrace, then did:
>>>
>>> $ dd if=/dev/zero of=foo bs=4k count=128 && sync
>>>
>>> to ensure that I knew merges would be happening. Output stats at the end:
>>>
>>> Total (sda):
>>> Reads Queued:           7,       44KiB  Writes Queued:         447,     7692KiB
>>> Read Dispatches:        7,       44KiB  Write Dispatches:      416,     7692KiB
>>> Reads Requeued:         0               Writes Requeued:         0
>>> Reads Completed:        7,       44KiB  Writes Completed:      435,     5864KiB
>>> Read Merges:            0,        0KiB  Write Merges:           23,      428KiB
>>> IO unplugs:            78               Timer unplugs:           0
>>>
>>> -- 
>>> Jens Axboe
>>>
>> First, Thanks your time!
>> If i understand correctly, the merge of your example is bio with
>> request, not request wiht request.  Yes or no?
>
>It is bio to request, correct. Request to request merges are relatively
>more rare.
>
>-- 
>Jens Axboe
>
Thanks very much, I know.

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