Re: [PATCH] deadline io scheduler

Andrew Morton
2002-09-26T07:07:47Z

Andrew Morton wrote:
>
> I'll test scsi now.
>

aic7xxx, Fujitsu "MAF3364L SUN36G" (36G SCA-2)

Maximum number of TCQ tags=253

fifo_batch time cat kernel/*.c (seconds)
64 58
32 54
16 20
8 58
4 1:15
2 53

Maximum number of TCQ tags=4

fifo_batch time cat kernel/*.c (seconds)
64 53
32 39
16 33
8 21
4 22
2 36
1 22

Maximum number of TCQ tags = 0:

fifo_batch time cat kernel/*.c (seconds)
64 22
32 10.3
16 10.5
8 5.5
4 3.2
2 1.9

I selected fifo_batch=16 and altered writes_starved and read_expires
again. They made no appreciable difference.

>From this I can only conclude that my poor little read was stuck
in the disk for ages while TCQ busily allowed new incoming writes
to bypass already-sent reads.

A dreadful misdesign. Unless we can control this with barriers,
and if Fujutsu is typical, TCQ is just uncontrollable. I, for
one, would not turn it on in a pink fit.