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Date: Fri, 09 Aug 2013 10:42:21 -0700
From: Dave Hansen <dave.hansen@intel.com>
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To: Jan Kara <jack@suse.cz>
CC: Andy Lutomirski <luto@amacapital.net>, linux-mm@kvack.org,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)
References: <cover.1375729665.git.luto@amacapital.net> <20130807134058.GC12843@quack.suse.cz> <520286A4.1020101@intel.com> <CALCETrXAz1fc7y07LhmxNh6zA_KZB4yv57NY2MrhUwKdkypB9w@mail.gmail.com> <20130808101807.GB4325@quack.suse.cz> <CALCETrX1GXr58ujqAVT5_DtOx+8GEiyb9svK-SGH9d+7SXiNqQ@mail.gmail.com> <20130808185340.GA13926@quack.suse.cz> <CALCETrVXTXzXAmUsmmWxwr6vK+Vux7_pUzWPYyHjxEbn3ObABg@mail.gmail.com> <5204229F.8000507@intel.com> <20130809075523.GA14574@quack.suse.cz>
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On 08/09/2013 12:55 AM, Jan Kara wrote:
> On Thu 08-08-13 15:58:39, Dave Hansen wrote:
>> > I was coincidentally tracking down what I thought was a scalability
>> > problem (turned out to be full disks :).  I noticed, though, that ext4
>> > is about 20% slower than ext2/3 at doing write page faults (x-axis is
>> > number of tasks):
>> > 
>> > http://www.sr71.net/~dave/intel/page-fault-exts/cmp.html?1=ext3&2=ext4&hide=linear,threads,threads_idle,processes_idle&rollPeriod=5
>> > 
>> > The test case is:
>> > 
>> > 	https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c
>   The reason is that ext2/ext3 do almost nothing in their write fault
> handler - they are about as fast as it can get. ext4 OTOH needs to reserve
> blocks for delayed allocation, setup buffers under a page etc. This is
> necessary if you want to make sure that if data are written via mmap, they
> also have space available on disk to be written to (ext2 / ext3 do not care
> and will just drop the data on the floor if you happen to hit ENOSPC during
> writeback).

I did try throwing a fallocate() in there to see if it helped.  It
didn't appear to help.  Should it have?