Re: O_DIRECT performance impact on 2.4.18 (was: Re: [PATCH] 2.5.14IDE 56)

[Andrea Arcangeli]

On Fri, May 10, 2002 at 12:15:19AM -0700, Andrew Morton wrote:
> Lincoln Dale wrote:
> > 
> > At 11:50 AM 9/05/2002 -0700, Andrew Morton wrote:
> > > >          /dev/md0 raid-0 with O_DIRECT:          91847kbyte/sec (2781usec
> > > >          /dev/md0
> > > raid-0:                                129455kbyte/sec (1978usec
> > > >          /dev/md0 raid-0 with O_NOCOPY:  195868kbyte/sec (1297usec
> > >
> > >hmm.  Why is O_DIRECT always the slowest?  (and it would presumably do
> > >even worse with an 8k transfer size).
> > 
> > i just reproduced the test to validate the data.  i'm using 8kbyte blocks here.
> > on kernel is 2.4.18, O_DIRECT is still the slowest.
> 
> 8k would rather disadvantage O_DIRECT.  It can't do readahead and it
> can't to write-behind.  It'll be forced to do tons of tiny I/Os.

yes, that's the main factor. Suggested read/write buffer size with
O_DIRECT are of the order of 512k at least (that's the high limit for a
single scsi dma request in 2.4), and 1M even better so you I/O pipeline
some more in the ll_rw_block layer.

Andrea