Re: Poor read performance when sequential write presents

[Andrew Morton]

"chen, xiangping" wrote:
> 
> Hi,
> 
> I did a IO test with one sequential read and one sequential write
> to different files. I expected somewhat similar throughput on read
> and write. But it seemed that the read is blocked until the write
> finishes. After the write process finished, the read process slowly
> picks up the speed. Is Linux buffer cache in favor of write? How
> to tune it?
> 

Reads and writes are very different beasts - writes deal with
the past and have good knowledge of what to do.  But reads
must predict the future.

You need to do two things:

1: Configure the device for a really big readahead window.

   Configuring readahead in 2.4 is a pig.  Try one of the
   following:

     echo file_readahead:N > /proc/ide/hda/settings   (N is kilobytes)
     blockdev --setra M /dev/hda                      (M is in 512 byte sectors)
     echo K > /prov/sys/vm/max-readahead              (K is in pages - 4k on x86)

   You'll find that one of these makes a difference.

2: Apply http://www.zip.com.au/~akpm/linux/patches/2.4/2.4.19-pre5/read-latency2.patch
   which will prevent reads from being penalised by writes.
   Or use a -ac kernel, which already has this patch.

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