Re: High read Latency test (Anticipatory I/O scheduler)

[Andrew Morton]

"John Chatelle" <johnch@xxxxxxxxxx> wrote:
>
> #StreamingRead.sh       --simple 4 line shell script:
>    while true
>    do
>      cat ../data/oneGBfile >/dev/null
>    done
> 
> #WHR.sh                -- simple 2 (or 3) line shell script.
>    StreamingRead.sh &
>    time find /usr/src/linux-2.4.20-18.7  -type f -exec cat \
>      '{}' ';' > /dev/null

Note that the latter test runs `cat' once per file: over ten thousand
times.  You should also test:

	time  (find /usr/src/linux-2.4.20-18.7 -type f | xargs cat > /dev/null )

which will run `cat' only some tens of times.

For the anticipatory scheduler, this makes a significant difference - the
run-cat-once-per-file problem has specific fixes and perhaps they're not
working right at present.

And yes, please describe your disk system, tell us the tag depth (if it is
scsi) and try Nick's patch, thanks.

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