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SubjectRe: RFC on io-stalls patch
On Tue, Jul 15, 2003 at 01:27:37PM +0200, Jens Axboe wrote:
> On Tue, Jul 15 2003, Alan Cox wrote:
> > On Maw, 2003-07-15 at 06:26, Jens Axboe wrote:
> > > Sorry, but I think that is nonsense. This is the way we have always
> > > worked. You just have to maintain a decent queue length still (like we
> > > always have in 2.4) and there are no problems.
> >
> > The memory pinning problem is still real - and always has been. It shows up
> > best not on IDE disks but large slow media like magneto opticals where you
> > can queue lots of I/O but you get 500K/second
>
> Not the same thing. On slow media, like dvd-ram, what causes the problem
> is that you can dirty basically all of the RAM in the system. That has
> nothing to do with memory pinned in the request queue.

you can trivially bound the amount of dirty memory to nearly 0% with the
bdflush sysctl. And the overkill size of the queue until pre3 could be
an huge VM overhead compared to the dirty memory on lowmem boxes,
example a 32/64M machine. So I disagree it's only a mistake of write
throttling that gives problems on slow media.

Infact I tend to think the biggest problem for slow media in 2.4 is the
lack of per spindle pdflush.

Andrea
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