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SubjectRe: Swapping algorithm

I'll ask my prof.


On Mon, 29 Apr 1996, J.J. Burgess wrote:

> On Tue, 23 Apr 1996, David S. Miller wrote:
> > Date: Mon, 22 Apr 96 18:26 BST
> > From: Jamie Lokier <>
> >
> > I like. How about: gather statistics about working sets in the usual
> > way, based on the usual page faults. Do it better, of course.
> > Occasionally, unmap a few random pages from random processes, so that
> > they trigger page faults and add to the statistics. The pages are still
> > in the page cache, of course, so it doesn't take long to map them back.
> > Although there is a speed penalty for this, the improved statistics, if
> > used well, mean that you are far less likely to swap out pages that are
> > actively in use. How else do you find the working set of a process
> > without swapping it all to disk?
> >
> > Or the heuristic could work like this. When we take a page out of a
> > processes VMA we add a "current-jiffies" marker to the vma when we do
> > this. The next time we take a fault in that vma we see how long the
> > jiffies have gone since we took that page away. Using this we decide
> > later on whether it is a good idea to steal from that vma. And we
> > re-juvenate these values or retest every so often.
> >
> > Later,
> > David S. Miller
> >
> >
> Here is a more considered view of how this might be done, its the
> most efficient and easiest way I can think of.
> Swapping handles 3 differnt things:
> 1. Clear out unused pages - init code etc.
> Done already (poorly) can be done slowly
> results in page out (not swap) of clean
> code segments.
> Speculative unpaging is great for this,
> every time memory starts getting short,
> or load gets very low we should look for
> these. When we actually need to swap, we
> look first at pages who were unmapped ageas
> ago and haven't been accessed since.
> 2. Get rid of most infrequently used pages.
> 3. Sequentially accessed data, too much to hold at once
> e.g. image processing, matrix operations
> Sometimes operations performed at a few lines of
> a picture, working data window slides down image
> Speculative unmapping copes with all this, and since a
> heavy paging currently results in much idle time,
> waiting on disc access, hopefully we'll have a lot
> of spare time to do this.
> Swapping policy: Swap pages who have been unmapped the longest,
> by including with all pages a time reference, in jiffies, which
> as well as the status (normal, unmapped, swapped) tells us when
> it was swapped out, swapped in or last accessed (from unmapping) or when unmapped.
> i.e. the last time it changed status.
> How do we do the unmapping?
> - Surely we can use most of the current swapping code, to unmap but
> not swap out the processes access to a page of memory, generating a
> page fault on access, where upon we give the page back and update
> our last_accessed time.
> How do we choose what to unmap?
> - ?
> We don't want to unmap pages too frequently, although the penalty in time
> of unmapping a page is surely a lot less than swapping.
> How often do we do the unmapping?
> - ?
> Perhaps all the time we're idle, the only penalty is an increase in the process
> execution time as it slowly gets its pages back. However if this was happening
> for every page the CPU accessed this wouldn't be good.
> Implementing a mechanism to back off unmapping frequently used pages is hard.
> If we unmap all memory with a (guess) penalty of 1ms overhead.
> 16M / 4k = 4096 pages. i.e. 4 seconds for whole sweep, this is a long time
> we must do it in an order fashion, a bit at a time.
> We can use the 'last time was known accessed' to determine what we unmap.
> Maybe we sweep the page tables and unmap these pages, these will then
> have there timers updated if and when they are again accessed.
> (A fixed number per sweep) Only used pages will get re-mapped, so after a while
> we'll just be unmapping frequently used pages - we must be careful not to do too
> much, however my machine has a large idle time normally. Perhaps we should
> decrease the number of pages scanned for when CPU is low.
> In addition to the above swapping policy, if we have a whole load of sequential
> pages all unused for a similarly long period of time we swap them all rather
> than just individually. How do we decide how close is close who knows?
> This will have to be tested and tuned. - Maybe we could unmap them all
> simultaneously, unmap an entire process address space at once maybe. Seems
> a bit rough on the process, not quite as bad as swapping it all though.
> Doing all the above will guarantee we don't swap out frequently used pages,
> although I don't have a heavy swapping system myself (16M Ram) so I don't
> have much experience of what people do in very bad swap situations.
> Really we ought to get some computer scientist to give some ideas, aren't you
> meant to cover such algorithms in your courses? Has anyone got any references?
> Or knows how other OS's do it any better?
> Jon Burgess -
> Thanks.
> .. . . . . . . . . . . . . . ..
> :: : : Jon Burgess 01223-461907 : : ::
> :: : : : ::
> :: : : : : : : : : : : : : : ::

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