linux-kernel - Re: RAID-1 performance under 2.4 and 2.6

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Date:	Wed, 16 Jul 2008 14:18:25 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Pádraig Brady <P@...igBrady.com>
CC:	Emmanuel Florac <eflorac@...ellique.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RAID-1 performance under 2.4 and 2.6

Pádraig Brady wrote:
> Chris Snook wrote:
>> Bill Davidsen wrote:
>>> Chris Snook wrote:
>>>> Emmanuel Florac wrote:
>>>>> I post there because I couldn't find any information about this
>>>>> elsewhere : on the same hardware ( Athlon X2 3500+, 512MB RAM, 2x400 GB
>>>>> Hitachi SATA2 hard drives ) the 2.4 Linux software RAID-1 (tested
>>>>> 2.4.32
>>>>> and 2.4.36.2, slightly patched to recognize the hardware :p) is way
>>>>> faster than 2.6 ( tested 2.6.17.13, 2.6.18.8, 2.6.22.16, 2.6.24.3)
>>>>> especially for writes. I actually made the test on several different
>>>>> machines (same hard drives though) and it remained consistent across
>>>>> the board, with /mountpoint a software RAID-1.
>>>>> Actually checking disk activity with iostat or vmstat shows clearly a
>>>>> cache effect much more pronounced on 2.4 (i.e. writing goes on much
>>>>> longer in the background) but it doesn't really account for the
>>>>> difference. I've also tested it thru NFS from another machine (Giga
>>>>> ethernet network):
>>>>>
>>>>> dd if=/dev/zero of=/mountpoint/testfile bs=1M count=1024
>>>>>
>>>>> kernel        2.4       2.6        2.4 thru NFS   2.6 thru NFS
>>>>>
>>>>> write        90 MB/s    65 MB/s      70 MB/s       45 MB/s
>>>>> read         90 MB/s    80 MB/s      75 MB/s       65 MB/s
>>>>>
>>>>> Duh. That's terrible. Does it mean I should stick to  (heavily
>>>>> patched...) 2.4 for my file servers or... ? :)
>>>>>
>>>> It means you shouldn't use dd as a benchmark.
>>>>
>>> What do you use as a benchmark for writing large sequential files or
>>> reading them, and why is it better than dd at modeling programs which
>>> read or write in a similar fashion?
>>>
>>> Media programs often do data access in just this fashion,
>>> multi-channel video capture, streaming video servers, and similar.
>>>
>> dd uses unaligned stack-allocated buffers, and defaults to block sized
>> I/O.  To call this inefficient is a gross understatement.  Modern
>> applications which care about streaming I/O performance use large,
>> aligned buffers which allow the kernel to efficiently optimize things,
>> or they use direct I/O to do it themselves, or they make use of system
>> calls like fadvise, madvise, splice, etc. that inform the kernel how
>> they intend to use the data or pass the work off to the kernel
>> completely.  dd is designed to be incredibly lightweight, so it works
>> very well on a box with a 16 MHz CPU.  It was *not* designed to take
>> advantage of the resources modern systems have available to enable
>> scalability.
>>
>> I suggest an application-oriented benchmark that resembles the
>> application you'll actually be using.
> 
> I was trying to speed up an app¹ I wrote which streams parts of a large file,
> to separate files, and tested your advice above (on ext3 on 2.6.24.5-85.fc8).
> 
> I tested reading blocks of 4096, both to stack and page aligned buffers,
> but there were negligible differences between the CPU usage between the
> aligned and non-aligned buffer case.
> I guess the kernel could be clever and only copy the page to userspace
> on modification in the page aligned case, but the benchmarks at least
> don't suggest this is what's happening?
> 
> What difference exactly should be expected from using page aligned buffers?
> 
> Note I also tested using mmap to stream the data, and there is a significant
> decrease in CPU usage in user and kernel space as expected due to the
> data not being copied from the page cache.
> 
> thanks,
> Pádraig.
> 
> ¹ http://www.pixelbeat.org/programs/dvd-vr/

Page alignment, by itself, doesn't do much, but it implies a couple of 
things:

1) cache line alignment, which matters more with some architectures than 
others

2) block alignment, which is necessary for direct I/O

You're on the right track with mmap, but you may want to use madvise() 
to tune the readahead on the pagecache.

-- Chris
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