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Message-ID: <50580B51.70300@kernel.dk>
Date: Tue, 18 Sep 2012 07:49:05 +0200
From: Jens Axboe <axboe@...nel.dk>
To: Jianpeng Ma <majianpeng@...il.com>
CC: tj <tj@...nel.org>, rostedt <rostedt@...dmis.org>,
fweisbec <fweisbec@...il.com>, mingo <mingo@...hat.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Li Zefan <lizf@...fujitsu.com>, Tao Ma <boyu.mt@...bao.com>
Subject: Re: Why blktrace didn't trace requests merge?
On 2012-09-18 02:30, Jianpeng Ma wrote:
> On 2012-09-18 02:27 Jens Axboe <axboe@...nel.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
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