[Gllug] Large Page Patch

[Huw Lynes]

On Wednesday 14 August 2002 08:53, Dave Jones wrote:
> On Wed, Aug 14, 2002 at 08:40:45AM +0100, Huw Lynes wrote:
>  > That makes sense. Could the 64k have been in reference to
>  > something else? Perhapse in accesing block devices?
>
> Possibly in reference to soft-page style pre-faulting/readahead.
>
After some googling around in lkml I found the following in a 
discussion on ext2 block-size and disk performance:

	You may want to try using raw I/O to fully characterize the behavior
	of you device.  I found that when I use raw I/O I could get very good
	performance characteristics for devices.  If you use dd with a raw
	device you can vary the block size to see what kind of performance you
	get out of different sizes.  This will completely bypass any affects
	of buffer cache to get you the performance of the disk in question.
	An example of this would be to run this sequence of commands noting
	the time it takes to run it (all transfers 100MB):

	time dd if=/dev/zero of=/dev/raw1 bs=512 count=204800
	time dd if=/dev/zero of=/dev/raw1 bs=1k  count=102400
	time dd if=/dev/zero of=/dev/raw1 bs=4k  count=25600
	...

	The standard Stephen Tweedie raw I/O will do up to 64KB chunks, beyond
	that you'll probably have to write one specific to your device (SGI
	has one for SCSI which I've gotten up to 1MB reads/writes).  Using dd
	doesn't necessarily show you your performance as most access patterns
	will not be completely sequential in nature, but you can figure out
	what your "sweet spot" is for your block size.

This was back in 2000 around about 2.2.13; 2.4 probably uses 64KB 
chunks by default for block device access. The talk of increasing it to 
1MB gels with what the engineer said. I guess what I need to do now is 
find out what chunk size 2.4 uses and wether or not it can be 
increased. Although I don't relish the thought of having to use raw I/O 
to get the performance I need.


-- 
| Huw Lynes               | The Moving Picture Company  |
| System Administrator    | 127 Wardour Street          |
|.........................| London, W1F 0NL             | 



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