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SubjectRe: Where is the performance bottleneck?
On Wed, Aug 31 2005, Holger Kiehl wrote:
> >>>Ok, I did run the following dd command in different combinations:
> >>>
> >>> dd if=/dev/zero of=/dev/sd?1 bs=4k count=5000000
> >>
> >>I think a bs of 4k is way too small and will cause huge CPU overhead.
> >>Can you try with something like 4M? Also, you can use /dev/full to avoid
> >>the pre-zeroing.
> >
> >That was my initial thought as well, but since he's writing the io side
> >should look correct. I doubt 8 dd's writing 4k chunks will gobble that
> >much CPU as to make this much difference.
> >
> >Holger, we need vmstat 1 info while the dd's are running. A simple
> >profile would be nice as well, boot with profile=2 and do a readprofile
> >-r; run tests; readprofile > foo and send the first 50 lines of foo to
> >this list.
> >
> Here vmstat for 8 dd's still with 4k blocksize:
>
> procs -----------memory---------- ---swap-- -----io---- --system--
> ----cpu----
> r b swpd free buff cache si so bi bo in cs us sy id
> wa
> 9 2 5244 38272 7738248 10400 0 0 3 11444 390 24 0 5
> 75 20
> 5 10 5244 30824 7747680 8684 0 0 0 265672 2582 1917 1 95
> 0 4
> 2 12 5244 30948 7747248 8708 0 0 0 222620 2858 292 0 33
> 0 67
> 4 10 5244 31072 7747516 8644 0 0 0 236400 3132 326 0 43
> 0 57
> 2 12 5244 31320 7747792 8512 0 0 0 250204 3225 285 0 37
> 0 63
> 1 13 5244 30948 7747412 8552 0 0 24 227600 3261 312 0 41
> 0 59
> 2 12 5244 32684 7746124 8616 0 0 0 235392 3219 274 0 32
> 0 68

[snip]

Looks as expected, nothing too excessive showing up. About 30-40% sys
time, but it should not bog the machine down that much.

> And here the profile output (I assume you meant sorted):

I did, thanks :)

> 3236497 total 1.4547
> 2507913 default_idle 52248.1875
> 158752 shrink_zone 43.3275
> 121584 copy_user_generic_c 3199.5789
> 34271 __wake_up_bit 713.9792
> 31131 __make_request 23.1629
> 22096 scsi_request_fn 18.4133
> 21915 rotate_reclaimable_page 80.5699
> 20641 end_buffer_async_write 86.0042
> 18701 __clear_user 292.2031

Nothing sticks out here either. There's plenty of idle time. It smells
like a driver issue. Can you try the same dd test, but read from the
drives instead? Use a bigger blocksize here, 128 or 256k.

You might want to try the same with direct io, just to eliminate the
costly user copy. I don't expect it to make much of a difference though,
feels like the problem is elsewhere (driver, most likely).

If we still can't get closer to this, it would be interesting to try my
block tracing stuff so we can see what is going on at the queue level.
But lets gather some more info first, since it requires testing -mm.

--
Jens Axboe

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