Re: Linux 2.6.29

Jeff Garzik
2009-04-03T03:05:38Z

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