Messages in this thread Patch in this message | | | Date | Wed, 08 Sep 2004 09:20:08 -0500 | From | Ray Bryant <> | Subject | Re: swapping and the value of /proc/sys/vm/swappiness |
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Marcelo Tosatti wrote:
> > Andrew, dirty_ratio and dirty_background_ratio (as low as 5% each) did not significantly > affect the amount of swapped out data, only a small effect on _how soon_ anonymous > memory was swapped out. >
I looked at the get_dirty_limits() code and for the test cases I was running, we have mapped > 90% of memory. So what will happen is that dirty_ratio will be thresholded at 5%, and background_ratio will be 1%. Changing values in /proc won't modify this at all (well, you could force background_ratio to 0%.)
It seems to me that the 5% number in there is more or less arbitrary. If we are on a big memory Altix (4 TB), 5% of memory would be 200 GB. That is a lot of page cache.
It seems get_dirty_limits() would be a lot simpler (and automatically scale as memory is mapped) if the limits were interpreted as being in terms of the amount of unmapped memory. A patch that implements this idea is attached. (Andrew -- if it comes to that I can submit this patch inline -- this is just for talking at the moment).
I'll run a few of the tests with this modified kernel and see if they are any different.
> And finally, Ray, the difference you see between 2.6.6 and 2.6.7 can be explained, > as noted by others in this thread, to vmscan.c changes (page replacement/scanning policy > changes were made).
Yep. I can probably live with those minor differences though. I would be happier if the system didn't swap anything at all for low values of swappiness, though.
> > Will continue with more tests tomorrow. >
-- Best Regards, Ray ----------------------------------------------- Ray Bryant 512-453-9679 (work) 512-507-7807 (cell) raybry@sgi.com raybry@austin.rr.com The box said: "Requires Windows 98 or better", so I installed Linux. ----------------------------------------------- Index: linux-2.6.9-rc1-mm3-kdb/mm/page-writeback.c =================================================================== --- linux-2.6.9-rc1-mm3-kdb.orig/mm/page-writeback.c 2004-09-03 10:18:57.000000000 -0700 +++ linux-2.6.9-rc1-mm3-kdb/mm/page-writeback.c 2004-09-07 14:46:24.000000000 -0700 @@ -135,32 +135,19 @@ static void get_dirty_limits(struct writeback_state *wbs, long *pbackground, long *pdirty) { - int background_ratio; /* Percentages */ - int dirty_ratio; - int unmapped_ratio; + int unmapped; long background; long dirty; struct task_struct *tsk; get_writeback_state(wbs); - unmapped_ratio = 100 - (wbs->nr_mapped * 100) / total_pages; - dirty_ratio = vm_dirty_ratio; - if (dirty_ratio > unmapped_ratio / 2) - dirty_ratio = unmapped_ratio / 2; + unmapped = total_pages - wbs->nr_mapped; - if (dirty_ratio < 5) - dirty_ratio = 5; + background = (dirty_background_ratio * unmapped) / 100; + dirty = (vm_dirty_ratio * unmapped) / 100; - /* - * Keep the ratio between dirty_ratio and background_ratio roughly - * what the sysctls are after dirty_ratio has been scaled (above). - */ - background_ratio = dirty_background_ratio * dirty_ratio/vm_dirty_ratio; - - background = (background_ratio * total_pages) / 100; - dirty = (dirty_ratio * total_pages) / 100; tsk = current; if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { background += background / 4;
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