> 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/
