On September 6, 2001 09:25 pm, Rik van Riel wrote:
> On Thu, 6 Sep 2001, Daniel Phillips wrote:
> > On September 6, 2001 06:57 pm, Rik van Riel wrote:
> > > On Thu, 6 Sep 2001, Daniel Phillips wrote:
> > >
> > > > Err, not quite the whole story. It is *never* right to leave the disk
> > > > sitting idle while there are dirty, writable IO buffers.
> > >
> > > Define "idle" ?
> >
> > Idle = not doing anything. IO queue is empty.
> >
> > > Is idle the time it takes between two readahead requests
> > > to be issued, delaying the second request because you
> > > just moved the disk arm away ?
> >
> > Which two readahead requests? It's idle.
>
> OK, in this case I disagree with you ;)
>
> Disk seek time takes ages, as much as 10 milliseconds.
>
> I really don't think it's good to move the disk arm away
> from the data we are reading just to write out this one
> disk block.
>
> Going 20 milliseconds out of our way to write out a single
> block really can't be worth it in any scenario I can imagine.
>
> OTOH, flushing out 64 or 128 kB at once (or some fraction of
> the inactive list, say 5%?) almost certainly is worth it in
> many cases.
Again, I have to ask, which reads are you interfering with? Ones that
haven't happened yet? Remember, the disk is idle. So *at worst* you are
going to get one extra seek before getting hit with the tidal wave of reads
you seem to be worried about. This simply isn't significant.
I've tested this, I know early writeout under light load is a win.
What we should be worrying about is how to balance reads against writes under
heavy load.
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This archive was generated by hypermail 2b29 : Fri Sep 07 2001 - 21:00:37 EST