Messages in this thread |  | | From | Daniel Phillips <> | Subject | Re: page_launder() on 2.4.9/10 issue | Date | Thu, 6 Sep 2001 21:45:35 +0200 |
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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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