On 02/01/14 22:32, Wolfgang Denk wrote:
Dear Stan,
In message <52C5A9AA.9090300@xxxxxxxxxxxxxxxxx> you wrote:
filesystem blocks and memory pages aren't necessarily 4K, though that
is the most common size.
Yes, they are necessarily 4K in Linux. Linux only supports page sized
BIO for consistency across the memory manager and IO subsystems. Most
architectures which Linux currently supports have hardware page sizes
greater than 4K, for instance IA64 supports 4k/8k/16k, even a 4GB page
size. But it was decided long ago to stick with 4K for a number of
reasons, one of these is stated above. For background on this Google is
your friend.
Well, you can tune the page size - and if you need no file system
support (like when implementing a RAID controller card) making the
page size exactly the same as the chunk size will allow for some nice
performance optimizations (as you can avoid a lot of large memcpy()
operations).
We did this (some 6 years ago) for the (then AMCC) PPC440SPe
processors; I can't find the old document any longer on APM's web
site, but there is still a copy here ([1]) which shows the effect.
I had the chance to work with those AMCC boards and I confirm that
aligning the chunk size on the (larger) PPC440 page size yielded some
impressive performance results. We were topping the hardware
capabilities. On the other hand, the entire kernel was built for this
particular hardware and I'm not sure how well tuning the system page
size would work on an Intel platform.
So yes, tuning the system page size can have considerable impact, but
only for special-purpose applications, and when optimising for large
sequential I/O (which appears to be how the RAID controller
manufacturers are testing / optimizing their systems).
Fact is, with a file system on top of the RAID array, and with our
typical work load of many very small files, A RAID6 with chunk size
16k will give much better results that with chunk size 64k - and
anything even bigger will be worse.
[1] ftp://ftp.denx.de/pub/demos/RAID-demo/doc/RAIDinLinux_PB_0529a.pdf
Best regards,
Wolfgang Denk
Regards,
Ben.
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