Re: high system cpu load during intense disk i/o

Rafał Bilski
2007-08-08T19:08:49Z

> Hello again,
Hi,
> I 'm now using libata on the same system described before (see attached
> dmesg.txt). When writing to both disks I think the problem is now worse
> (pata_oprof_bad.txt, pata_vmstat_bad.txt), even the oprofile script needed
> half an hour to complete! For completion I also attach the same tests when I
> write to only one disk (pata_vmstat_1disk.txt, pata_oprof_1disk.txt), whence
> everything is normal.
>
> FWIW, libata did not give me any performance benefit, 20MB/s is again the peak
> hdparm reports.

This OProfile thing is extremly not usefull in this case. It says that Your
system is using 25% CPU time for no-op loops, but it doesn't say why. Your
system really isn't very busy. Look here:
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
2 2 0 225352 5604 91700 0 0 18112 1664 28145 6315 29 71 0 0
2 2 0 225360 5604 91700 0 0 18496 1664 27992 6358 30 70 0 0
1 2 0 225360 5604 91700 0 0 18432 1472 28511 6315 28 72 0 0
1 2 0 225360 5604 91700 0 0 18240 1536 28031 6153 31 69 0 0
+ video 720x576 25fps yuv stream over PCI. And system is fully responsive. Of
course programs which need disk access must wait a bit longer, but later are
running fine.
I don't have disks so fast like Yours and I can't do destructive write test.
First disk:
1 1 0 241848 7312 100768 0 0 27712 0 927 1270 29 13 0 58
1 1 0 241052 7580 100896 0 0 4612 4676 519 702 34 12 0 54
Second disk:
0 1 0 237752 7268 100980 0 0 6464 0 468 583 37 10 0 53
0 1 0 241060 7532 100884 0 0 1728 1728 465 578 31 9 0 60
Both:
0 2 0 241592 7384 100776 0 0 33024 0 905 1415 33 16 0 51
1 2 0 240804 7528 100884 0 0 6848 6848 642 780 38 10 0 52
So sda + sdc = both.

Your single disk:
0 1 0 128804 19620 82484 0 0 0 21120 335 675 0 4 0 96
Both:
5 2 0 168480 10972 47152 0 0 0 16000 252 470 22 78 0 0
I would expect 2*21k, but we have only 2*8k and it is lower then single disk
case. Of course this math isn't moving us forward. Only thing which would
help would be function call trace as Andrew wrote. Which function is calling
delay_tsc()? Is it calling it often or once but with long delay?
So far it looks like some kind of hardware limit for me. Do You have any
options in BIOS which can degrade PCI or disk performance?
>
> Thanks,
> Dimitris
Rafał

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