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SubjectRe: Where is the performance bottleneck?
On Wed, 31 Aug 2005, Jens Axboe wrote:

> On Wed, Aug 31 2005, Vojtech Pavlik wrote:
>> On Tue, Aug 30, 2005 at 08:06:21PM +0000, Holger Kiehl wrote:
>>>>> How does one determine the PCI-X bus speed?
>>>>
>>>> Usually only the card (in your case the Symbios SCSI controller) can
>>>> tell. If it does, it'll be most likely in 'dmesg'.
>>>>
>>> There is nothing in dmesg:
>>>
>>> Fusion MPT base driver 3.01.20
>>> Copyright (c) 1999-2004 LSI Logic Corporation
>>> ACPI: PCI Interrupt 0000:02:04.0[A] -> GSI 24 (level, low) -> IRQ 217
>>> mptbase: Initiating ioc0 bringup
>>> ioc0: 53C1030: Capabilities={Initiator,Target}
>>> ACPI: PCI Interrupt 0000:02:04.1[B] -> GSI 25 (level, low) -> IRQ 225
>>> mptbase: Initiating ioc1 bringup
>>> ioc1: 53C1030: Capabilities={Initiator,Target}
>>> Fusion MPT SCSI Host driver 3.01.20
>>>
>>>> To find where the bottleneck is, I'd suggest trying without the
>>>> filesystem at all, and just filling a large part of the block device
>>>> using the 'dd' command.
>>>>
>>>> Also, trying without the RAID, and just running 4 (and 8) concurrent
>>>> dd's to the separate drives could show whether it's the RAID that's
>>>> slowing things down.
>>>>
>>> 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
1 13 5244 30948 7747940 8568 0 0 0 228020 3394 296 0 37 0 63
0 14 5244 31196 7747680 8624 0 0 0 232932 3389 300 0 32 0 68
3 12 5244 31072 7747904 8536 0 0 0 233096 3545 312 0 33 0 67
1 13 5244 31072 7747852 8520 0 0 0 226992 3381 290 0 31 0 69
1 13 5244 31196 7747704 8396 0 0 0 230112 3372 265 0 28 0 72
0 14 5244 31072 7747928 8512 0 0 0 240652 3491 295 0 33 0 67
3 13 5244 31072 7748104 8608 0 0 0 222944 3433 269 0 27 0 73
1 13 5244 31072 7748000 8508 0 0 0 207944 3470 294 0 28 0 72
0 14 5244 31072 7747980 8528 0 0 0 234608 3496 272 0 31 0 69
2 12 5244 31196 7748148 8496 0 0 0 228760 3480 280 0 28 0 72
0 14 5244 30948 7748568 8620 0 0 0 214372 3551 302 0 29 0 71
1 13 5244 31072 7748392 8524 0 0 0 226732 3494 284 0 29 0 71
0 14 5244 31072 7748004 8640 0 0 0 229628 3604 273 0 26 0 74
1 13 5244 30948 7748392 8660 0 0 0 212868 3563 266 0 28 0 72
1 13 5244 30948 7748600 8520 0 0 0 228244 3568 294 0 30 0 70
1 13 5244 31196 7748228 8416 0 0 0 221692 3543 258 0 27 0 73
1 13 5244 31072 7748192 8520 0 0 0 241040 3983 330 0 25 0 74
1 13 5244 31196 7748288 8560 0 0 0 217108 3676 276 0 28 0 72
.
.
.
This goses on up to the end.
.
.
.
0 3 5244 825096 6949252 8596 0 0 0 241244 2683 223 0 7 71 22
0 2 5244 825108 6949252 8596 0 0 0 229764 2683 214 0 7 73 20
0 3 5244 826348 6949252 8596 0 0 0 116840 2046 450 0 4 71 26
0 3 5244 826976 6949252 8596 0 0 0 141992 1887 97 0 4 73 23
0 3 5244 827100 6949252 8596 0 0 0 137716 1871 93 0 4 70 26
0 3 5244 827100 6949252 8596 0 0 0 137032 1894 96 0 4 75 21
0 3 5244 827224 6949252 8596 0 0 0 131332 1860 288 0 4 73 23
0 1 5244 1943732 5833756 8620 0 0 0 72404 1560 481 0 24 61 16
0 2 5244 1943732 5833756 8620 0 0 0 71680 1450 60 0 2 61 38
0 2 5244 1943736 5833756 8620 0 0 0 71680 1464 70 0 2 52 46
0 2 5244 1943736 5833756 8620 0 0 0 66560 1436 66 0 2 50 48
0 2 5244 1943984 5833756 8620 0 0 0 71680 1454 72 0 2 50 48
0 2 5244 1943984 5833756 8620 0 0 0 71680 1450 70 0 2 50 48
1 0 5244 2906484 4872176 8612 0 0 0 12760 1240 321 0 13 68 19
0 0 5244 3306732 4472300 8580 0 0 0 0 1109 31 0 9 91 0
0 0 5244 3306732 4472300 8580 0 0 0 0 1008 22 0 0 100 0

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

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
13562 __block_write_full_page 18.4266
12981 test_set_page_writeback 47.7243
10772 kmem_cache_free 96.1786
10216 unlock_page 159.6250
9492 free_hot_cold_page 32.9583
9478 add_to_page_cache 45.5673
9117 page_waitqueue 81.4018
8671 drop_buffers 38.7098
8584 __set_page_dirty_nobuffers 31.5588
8444 release_pages 23.9886
8204 scsi_dispatch_cmd 14.2431
8191 buffered_rmqueue 11.6349
7966 page_referenced 22.6307
7093 generic_file_buffered_write 4.1431
6953 __pagevec_lru_add 28.9708
6740 __alloc_pages 5.6926
6369 __end_that_request_first 11.7077
5940 dnotify_parent 30.9375
5880 kmem_cache_alloc 91.8750
5797 submit_bh 19.0691
4720 find_lock_page 21.0714
4612 __generic_file_aio_write_nolock 4.8042
4559 __do_softirq 20.3527
4337 end_page_writeback 54.2125
4090 create_empty_buffers 25.5625
3985 bio_alloc_bioset 9.2245
3787 mempool_alloc 12.4572
3708 set_page_refs 231.7500
3545 __block_commit_write 17.0433
3037 system_call 23.1832
2968 zone_watermark_ok 15.4583
2966 cond_resched 26.4821
2828 generic_make_request 4.7770
2766 __mod_page_state 86.4375
2759 fget_light 15.6761
2692 test_clear_page_dirty 11.2167
2523 vfs_write 8.2993
2406 generic_file_aio_write_nolock 15.0375
2335 bio_put 36.4844
2287 bad_range 23.8229

Under ftp://ftp.dwd.de/pub/afd/linux_kernel_debug/ I put the full vmstat
and profile output (also with -v). There is also dmesg and my kernel.config
from this system.

I will also do some test with 4M instead of 4k and as Al Boldi hinted
do a test together with some CPU load.

Thanks,
Holger

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