Messages in this thread | | | Subject | Re: [PATCH] Use extents for recording what swap is allocated. | From | Nigel Cunningham <> | Date | Wed, 25 Oct 2006 08:30:27 +1000 |
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Hi.
On Wed, 2006-10-25 at 00:19 +0200, Pavel Machek wrote: > Hi! > > > > > > Switch from bitmaps to using extents to record what swap is allocated; > > > > > they make more efficient use of memory, particularly where the allocated > > > > > storage is small and the swap space is large. > > > > > > > > As I said before, I like the overall idea, but I have a bunch of > > > > comments. > > > > > > Okay, if Rafael likes it... lets take a look. > > > > > > First... what is the _worst case_ overhead? AFAICT extents are very > > > good at the best case, but tend to suck for the worst case...? > > > > 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).
Regards,
Nigel
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