Messages in this thread | | | Subject | Re: [RFC][PATCH 0/2] Swap token re-tuned | From | Peter Zijlstra <> | Date | Mon, 02 Oct 2006 10:14:33 +0200 |
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On Mon, 2006-10-02 at 00:59 -0700, Andrew Morton wrote: > On Mon, 02 Oct 2006 09:35:52 +0200 > Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > On Sun, 2006-10-01 at 15:56 -0700, Andrew Morton wrote: > > > On Sat, 30 Sep 2006 00:11:51 +0530 > > > Ashwin Chaugule <ashwin.chaugule@celunite.com> wrote: > > > > > > PATCH 2: > > > > > > > > Instead of using TIMEOUT as a parameter to transfer the token, I think a > > > > better solution is to hand it over to a process that proves its > > > > eligibilty. > > > > > > > > What my scheme does, is to find out how frequently a process is calling > > > > these functions. The processes that call these more frequently get a > > > > higher priority. > > > > The idea is to guarantee that a high priority process gets the token. > > > > The priority of a process is determined by the number of consecutive > > > > calls to swap-in and no-page. I mean "consecutive" not from the > > > > scheduler point of view, but from the process point of view. In other > > > > words, if the task called these functions every time it was scheduled, > > > > it means it is not getting any further with its execution. > > > > > > > > This way, its a matter of simple comparison of task priorities, to > > > > decide whether to transfer the token or not. > > > > > > Does this introduce the possibility of starvation? Where the > > > fast-allocating process hogs the system and everything else makes no > > > progress? > > > > I tinkered with this a bit yesterday, and didn't get good results for: > > mem=64M ; make -j5 > > > > -vanilla: 2h32:55
Command being timed: "make -j5" User time (seconds): 2726.81 System time (seconds): 2266.85 Percent of CPU this job got: 54% Elapsed (wall clock) time (h:mm:ss or m:ss): 2:32:55 Average shared text size (kbytes): 0 Average unshared data size (kbytes): 0 Average stack size (kbytes): 0 Average total size (kbytes): 0 Maximum resident set size (kbytes): 0 Average resident set size (kbytes): 0 Major (requiring I/O) page faults: 269956 Minor (reclaiming a frame) page faults: 8699298 Voluntary context switches: 414020 Involuntary context switches: 242365 Swaps: 0 File system inputs: 0 File system outputs: 0 Socket messages sent: 0 Socket messages received: 0 Signals delivered: 0 Page size (bytes): 4096 Exit status: 0
> > -swap-token: 2h41:48
Command being timed: "make -j5" User time (seconds): 2720.54 System time (seconds): 2428.60 Percent of CPU this job got: 53% Elapsed (wall clock) time (h:mm:ss or m:ss): 2:41:48 Average shared text size (kbytes): 0 Average unshared data size (kbytes): 0 Average stack size (kbytes): 0 Average total size (kbytes): 0 Maximum resident set size (kbytes): 0 Average resident set size (kbytes): 0 Major (requiring I/O) page faults: 281943 Minor (reclaiming a frame) page faults: 8692417 Voluntary context switches: 421770 Involuntary context switches: 241323 Swaps: 0 File system inputs: 0 File system outputs: 0 Socket messages sent: 0 Socket messages received: 0 Signals delivered: 0 Page size (bytes): 4096 Exit status: 0
> > various other attempts at tweaking the code only made it worse. (will > > have to rerun these test, but a ~3h test is well, a 3h test ;-) > > I don't think that's a region of operation where we care a great deal. > What was the average CPU utlisation? Only a few percent.
~50%, its a slow box this, a p3-550.
> It's just thrashing too much to bother optimising for. Obviously we want > it to terminate in a sane period of time and we'd _like_ to improve it. > But I think we'd accept a 10% slowdown in this region of operation if it > gave us a 10% speedup in the 25%-utilisation region. > > IOW: does the patch help mem=96M;make -j5??
Will kick off some test later today.
> > Being frustrated with these results - I mean the idea made sense, so > > what is going on - I came up with this answer: > > > > Tasks owning the swap token will retain their pages and will hence swap > > less, other (contending) tasks will get less pages and will fault more > > frequent. This prio mechanism will favour exactly those tasks not > > holding the token. Which makes for token bouncing. > > OK. > > (We need to do something with > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.18/2.6.18-mm2/broken-out/mm-thrash-detect-process-thrashing-against-itself.patch, > btw. Has been in -mm since March and I'm still waiting for some benchmarks > which would justify its inclusion..)
Hmm, benchmarks, I need VM benchmarks for my page replacment work too ;-)
Perhaps I can create a multi-threaded progamm that knows a few patterns.
> > The current mechanism seemingly assigns the token randomly (whomever > > asks while not held gets it - and the hold time is fixed), however this > > change in paging behaviour (holder less, contenders more) shifts the > > odds in favour of one of the contenders. Also the fixed holding time > > will make sure the token doesn't get released too soon and can make some > > progress. > > > > So while I agree it would be nice to get rid of all magic variables > > (holding time in the current impl) this proposed solution hasn't > > convinced me (for one it introduces another). > > > > (for the interrested, the various attempts I tried are available here: > > http://programming.kicks-ass.net/kernel-patches/swap_token/ ) > > OK, thanks or looking into it. I do think this is rich ground for > optimisation.
Given the amazing reduction in speed I accomplished yesterday (worst was 3h09:02), I'd say we're not doing bad, but yeah, I too think there is room for improvement.
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