Re: [resent PATCH] Re: very slow parallel read performance

Roger Larsson
2001-08-25T18:02:46Z

On Saturdayen den 25 August 2001 13:49, Gérard Roudier wrote:
> On Sat, 25 Aug 2001, Roger Larsson wrote:

> > Where did the seek time go? And rotational latency?
>
> They just go away, for the following reasons:
>
> - For random IOs, IOs are rather seek time bounded.
> Tagged commands may help here.

> - For sequential IOs, the drive is supposed to prefetch data.
>

Ok, the disk does buffering itself (but it has less memory than . But will it
do read ahead when
there is a request to do something else waiting?
Will it continue to read when it should have moved?
(Note: lets assume that the disk are never out of requests)
When it moves. The arm has to move and the platter will rotate to the right
spot - lets see. You can start reading directly when you are at the right
track. It can be stuff needed later... (ok, latency can be ignored)

> > This is mine (a IBM Deskstar 75GXP)
> > Sustained data rate (MB/sec) 37
> > Seek time (read typical)
> > Average (ms) 8.5
> > Track-to-track (ms) 1.2
> > Full-track (ms) 15.0
> > Data buffer 2 MB
> > Latency (calculated 7200 RPM) 4.2 ms
> >
> > So sustained data rate is 37 MB/s
> >
> > > hdparm -t gives around 35 MB/s
> >
> > best I got during testing or real files is 32 MB/s

Note: the 32 MB/s is for reading two big file sequentially.
(first one then the other)
If I instead do a diff the two files the throughput drops
2.4.0 gave around 15 MB/s
2.4.7 gave only 11 MB/s
2.4.8-pre3 give around 25 MB/s (READA higher than normal)
2.4.9-pre2 gives 11 MB/s
2.4.9-pre4 gives 12 MB/s
My patch against 2.4.8-pre1 gives 28 MB/s

/RogerL

--
Roger Larsson
Skellefteå
Sweden