linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 02 Apr 2009 23:05:22 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Linus Torvalds wrote:
> 
> On Thu, 2 Apr 2009, Jeff Garzik wrote:
>> The most interesting thing I found:  the SSD does 80 MB/s for the first ~1 GB
>> or so, then slows down dramatically.  After ~2GB, it is down to 32 MB/s.
>> After ~4GB, it reaches a steady speed around 23 MB/s.
> 
> Are you sure that isn't an effect of double and triple indirect blocks 
> etc? The metadata updates get more complex for the deeper indirect blocks.

> Or just our page cache lookup? Maybe our radix tree thing hits something 
> stupid. Although it sure shouldn't be _that_ noticeable.

Indirect block overhead increased as the file grew to 23 GB, I'm sure...

I should probably re-test pre-creating the file, _then_ running 
overwrite.c.  That would at least guarantee the filesystem isn't 
allocating new blocks and metadata.

I was really surprised the performance was so high at first, then fell 
off so dramatically, on the SSD here.

Unfortunately I cannot trash these blkdevs, so the raw blkdev numbers 
are not immediately measurable.


>> There is a similar performance fall-off for the Seagate, but much less
>> pronounced:
>> 	After 1GB:	52 MB/s
>> 	After 2GB:	44 MB/s
>> 	After 3GB:	steady state
> 
> That would seem to indicate that it's something else than the disk speed. 
> 
>> There appears to be a small increase in system time with "-f" (use fadvise),
>> but I'm guessing time(1) does not really give a good picture of overall system
>> time used, when you include background VM activity.
> 
> It would also be good to just compare it to something like
> 
> 	time sh -c "dd + sync"

I'll add that to the next run...

	Jeff


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