Re: 6.0: Unable to make disks 100% busy in file system reads.

Julian Elischer wrote:

> On my 4.x systems, the following comand makes disks go about 100% 
> (well, 98%) busy
> (measured by systat -vmstat).
>
> tar cf /dev/null /usr
>
> I know that some versions of  tar recognise /dev/null as an output 
> device and
> cheat, so to be sure I confirmed that
>
> tar cf -  /usr | dd of=/dev/null bs=128k
>
> has the same result  (IDE drive).
>
> The same command run on 6.0 has difficulty keeping the drives 70% busy.
> (though for sume unknow reason I have seen it get to 87% for up to
> 10 or 15 seconds at a time).
>
> measured by gstat AND systems -vmstat.
>
> cpu usage at the time:
>
> 21.0%Sys   2.3%Intr  0.5%User  0.0%Nice 76.2%Idl
> 17.9%Sys   2.0%Intr  0.5%User  0.0%Nice 79.5%Idl
> it is noticable that the times that the disk usage goes higher (e.g. 87%)
> the system idle time is also higher, and sys time drops to about 6% so
> I am presuming it is a set of large files being traversed at that time.
> Softupdates is NOT enabled.
>
> Now if I start TWO of the work processses, the drive usage climbs to a
> pretty permanent 98% which is quite acceptable. So, it's not geom,
> at least, not in any direct manner.
>
> The interesting part is that 4.11 is able to force this disk usage 
> with just one
> work process.
>
> it seems to be something to do with the speed of the return 
> information for the read
> from disk.. Some scheduler interaction possibly, along with some side 
> effects of the
> new.


.. queueing behaviour.

> I've been looking at the way that the scheduling works and not seen 
> anything that
> really stands out.. If anyone has any ideas of other things to look at
> I'm all ears..


for example.. is the sequential lookahead working as well in the new 
world...

>
>
> Julian
>
> _______________________________________________
> freebsd-current_at_freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-current
> To unsubscribe, send any mail to 
> "freebsd-current-unsubscribe_at_freebsd.org"