Re: [PATCH] Use extents for recording what swap is allocated.

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > > That's right. In using this, we're relying on the fact that the swap
> > > allocator tries to act sensibly. I've only seen worse case performance
> > > when a user had two swap devices with the same priority (striped), but
> > > that was a bug. :)
> > 
> > Ok, but if the allocator somehow manages to stripe between two swap
> > devices, what happens?
> > 
> > IIRC original code was something like .1% overhead (8bytes per 4K, or
> > something?), bitmaps should be even better. If it is 1% in worst case,
> > that's probably okay, but it would be bad if it had overhead bigger
> > than 10times original code (worst case).
> 
> With the code I have in Suspend2 (which is what I'm working towards),
> the value includes the swap_type, so there's no overlap. Assuming the
> swap allocator does it's normal thing and swap allocated is contiguous,
> you'll probably end up with two extents: one containing the swap
> allocated on the first device, and the other containing the swap
> allocated on the second device. So (with the current version), striping
> would use 6 * sizeof(unsigned long) instead of 3 * sizeof(unsigned
> long).

And now, can you do same computation assuming the swap allocator goes
completely crazy, and free space is in 1-page chunks?

In particular, how much swap space can we have before we run out of
low memory? What is the overhead compared to bitmaps?
									Pavel
-- 
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