Re: Comment on patch to remove nr_async_pages limit

[Mike Galbraith]

On Tue, 5 Jun 2001, Marcelo Tosatti wrote:

> On Tue, 5 Jun 2001, Mike Galbraith wrote:
>
> > On Mon, 4 Jun 2001, Marcelo Tosatti wrote:
> >
> > > Zlatko,
> > >
> > > I've read your patch to remove nr_async_pages limit while reading an
> > > archive on the web. (I have to figure out why lkml is not being delivered
> > > correctly to me...)
> > >
> > > Quoting your message:
> > >
> > > "That artificial limit hurts both swap out and swap in path as it
> > > introduces synchronization points (and/or weakens swapin readahead),
> > > which I think are not necessary."
> > >
> > > If we are under low memory, we cannot simply writeout a whole bunch of
> > > swap data. Remember the writeout operations will potentially allocate
> > > buffer_head's for the swapcache pages before doing real IO, which takes
> > > _more memory_: OOM deadlock.
> >
> > What's the point of creating swapcache pages, and then avoiding doing
> > the IO until it becomes _dangerous_ to do so?
>
> Its not dangerous to do the IO. Now it _is_ dangerous to do the IO without
> having any sane limit on the amount of data being written out at the same
> time.

Yes.  If we start writing out sooner, we aren't stuck with pushing a
ton of IO all at once and can use prudent limits.  Not only because of
potential allocation problems, but because our situation is changing
rapidly so small corrections done often is more precise than whopping
big ones can be.

> > That's what we're doing right now.  This is a problem because we
> > guarantee it will become one.
>
> Its not really about swapcache pages --- its about anonymous memory.

(swapcache is the biggest pain in the butt for the portion of the spetrum
I'm hammering on though)

> If you're memory is full of anonymous data, you have to push some of this
> data to disk. (conceptually it does not really matter if its swapcache or
> not, think about anonymous memory)
>
> > We guarantee that the pagecache will become almost pure swapcache by
> > delaying the writeout so long that everything else is consumed.
>
> Exactly. And when we reach a low watermark of memory, we start writting
> out the anonymous memory.
>
> > In experiments, speeding swapcache pages on their way helps.  Special
> > handling (swapcache bean counting) also helps. (was _really ugly_ code..
> > putting them on a seperate list would be a lot easier on the stomach:)
>
> I agree that the current way of limiting on-flight swapout can be changed
> to perform better.
>
> Removing the amount of data being written to disk when we have a memory
> shortage is not nice.

Here, that doesn't make any real difference.  We can have too many pages
completing IO too late or too few.. problem is that they start coming
out of the pipe too late.  I'd rather see my poor disk saturated than
partly idle when my box is choking on dirtclods ;-)

	-Mike