Date:	Wed, 16 Jul 2008 15:52:32 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Chris Snook <csnook@...hat.com>
CC:	Emmanuel Florac <eflorac@...ellique.com>,
	linux-kernel@...r.kernel.org
Subject: Re: RAID-1 performance under 2.4 and 2.6

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/
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