Date: Sun, 31 Jul 2005 14:04:46 -0700 From: Julian Elischer <julian@elischer.org> To: Julian Elischer <julian@elischer.org> Cc: FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: 6.0: Unable to make disks 100% busy in file system reads. Message-ID: <42ED3CEE.2090803@elischer.org> In-Reply-To: <42ED3ADE.9040508@elischer.org> References: <42ED3ADE.9040508@elischer.org>
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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@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to > "freebsd-current-unsubscribe@freebsd.org"
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