linux-kernel - Re: A thought about IO scheduler in linux kernel for SSD

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Message-ID: <CAK2Ky6xRDcePGWB3Je7_5U5B+qq2Hx4Nv61VOcX3isL8hy7ynw@mail.gmail.com>
Date:	Fri, 25 Oct 2013 13:00:27 +0800
From:	韩磊 <bonben1989@...il.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Jan Kara <jack@...e.cz>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: A thought about IO scheduler in linux kernel for SSD

Thanks for your thought!  It may not make much sense. Because I think
the probability of two bios have the same start sector and the
situation mentioned by Ming Lei is too low.
Thanks ,
Bonben

2013/10/25 Ming Lei <tom.leiming@...il.com>:
> On Wed, Oct 23, 2013 at 6:59 PM, Jan Kara <jack@...e.cz> wrote:
>> On Wed 23-10-13 08:47:44, 韩磊 wrote:
>>> Nowadays,the IO schedulers in linux kernel have four types:
>>>
>>> deadline,noop,Anticiptory and CFQ.CFQ is the default scheduler.But CFQ is
>>> not a good scheduler for SSD,dealine may be a good choice.
>>
>>> When deadline runs,it has a mount of computation about merging and
>>> sorting.Merge has three types: front_merge,no_merge and back_merge.
>>> Why don't have  another type: merge based same sector.For example,it have
>>> two bios in a request list,theyboth have the same bi->sector,the bi->size
>>> maybe not equal. Whether can we put the latter bio replace the former?What
>>> do you find that significant?Or the other levels in OS has finished this
>>> function?
>>   That doesn't make much sense to me. If there are two bios in flight for
>> some sector, results are undefined. Thus we usually avoid such situation
>> (usually we want to have defined contents of the disk :). The exclusion is
>> usually achieved at higher level using page locking etc. So adding code
>> speeding up such requests doesn't seem worth it.
>
> The situation might be triggered when same file is read from two tasks,
> one is read via page cache, and another one is read by O_DIRECT.
>
> But still not sure if that makes sense.
>
> Thanks,
> --
> Ming Lei
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