Martin A. Fink wrote:
> Compared to ICH6R with AHCI OFF the only difference I can see is that with
> AHCI the system seems to reac much faster on keyboard events and screen
> redraw seems to be as fast as normal. It looks like that CPU usage has not
> decreased that dramatically as I would have expected it.
>
> Thus I did a small calculation:
> Assuming that the processor gives workloads of (a) 1B (b) 1kB (c) 64kB to the
> DMA controller in AHCI mode to write 45 MB/s to disk, I calculate for 10% CPU
> time usage of the 3.2 GHz Pentium
> (a) 10% * 3.2GHz / 45M calls = 7.3 CPU cycles per 1B call to DMA
> (b) 10% * 3.2GHz / 45k calls = 7.4E+03 CPU cycles per 1kB call to DMA
> (c) 10% * 3.2GHz / 720 calls = 4.8E+05 CPU cycles per 64kB call to DMA
>
> For me (a) looks reasonable (some overhead per byte), but stupid - if
> implemented. Giving bigger packages like (b) and (c) looks better to me, but
> then I can't understand that huge overhead (1E3 to 1E5 cpu cycles per
> package) for one package.
>
> Is this normal or do I still have something wrong in my system?
It's not ata_piix or ahci that's eating up your cpu cycles. It's
memcpy from your user program to kernel buffer.
[root]# dd if=/dev/zero of=/dev/sda bs=1M &
[1] 2649
[root]# vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 3 0 6216 477328 3660 0 0 2579 31377 955 1380 2 8 38 51
0 3 0 6008 477616 3680 0 0 0 55296 404 170 0 14 0 86
0 3 0 5772 477808 3640 0 0 0 73728 392 207 0 18 0 82
0 3 0 5896 477680 3656 0 0 0 74240 394 207 0 18 0 82
0 3 0 6084 477520 3656 0 0 0 73728 393 205 0 18 0 82
1 2 0 6652 477204 3668 0 0 0 57440 401 197 0 16 0 84
0 3 0 6136 477496 3664 0 0 0 72168 394 195 0 17 0 83
0 3 0 6320 477316 3644 0 0 0 73728 392 207 0 19 0 81
[root]# dd if=/dev/zero of=/dev/sda bs=1M oflag=direct &
[1] 2657
[root]# vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 1 0 494568 224 3836 0 0 835 11149 480 454 1 3 78 18
0 1 0 494568 224 3836 0 0 0 70656 406 146 0 2 0 98
0 1 0 494568 224 3840 0 0 0 69632 393 144 0 1 0 99
0 1 0 494568 232 3832 0 0 0 69680 396 152 0 2 0 98
0 1 0 494568 232 3840 0 0 0 68608 392 142 0 1 0 99
1 1 0 494568 232 3840 0 0 0 69632 395 144 0 2 0 98
0 1 0 494576 232 3840 0 0 0 69632 393 143 0 2 0 98
0 1 0 494576 232 3840 0 0 0 70656 395 144 0 1 0 99
0 1 0 494576 232 3840 0 0 0 70656 394 148 0 2 0 98
--
tejun
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