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Con Kolivas wrote: > I hate to resuscitate this old thread, sorry but I'm still not sure we > resolved it and I want to make sure this issue isn't here as I see it. > OK, reclaim is slightly different. > On Saturday 08 April 2006 11:25, Nick Piggin wrote: > >>Con Kolivas wrote: >> >>>Ok. I think I presented enough information for why I thought >>>zone_watermark_ok would fail (for ZONE_DMA). With 16MB ZONE_DMA and a >>>vmsplit of 3GB we have a lowmem_reserve of 12MB. It's pretty hard to keep >>>that much ZONE_DMA free, I don't think I've ever seen that much free on >>>my ZONE_DMA on an ordinary desktop without any particular ZONE_DMA users. >>>Changing the tunable can make the lowmem_reserve larger than ZONE_DMA is >>>on any vmsplit too as far as I understand the ratio. >> >>Umm, for ZONE_DMA allocations, ZONE_DMA isn't a lower zone. So that >>12MB protection should never come into it (unless it is buggy?). > > > An i386 pc with a 3GB split will have approx > > 4000 pages ZONE_DMA > > and lowmem reserve will set lowmem reserve to approx > > 0 0 3000 3000 > > So if we call zone_watermark_ok with zone of ZONE_DMA and a classzone_idx of a > ZONE_NORMAL we will fail a zone_watermark_ok test almost always since it's > almost impossible to have 3000 free ZONE_DMA pages. I believe it can happen > like this: > > In balance_pgdat (vmscan.c:1116) if we end up with end_zone being a > ZONE_NORMAL zone, then during the scan below we (vmscan.c:1137) iterate over > all zones from 0 to end_zone and (vmscan.c:1147) we end up calling > > if (!zone_watermark_ok(zone, order, zone->pages_high, end_zone, 0)) > > which would now call zone_watermark_ok with zone being a ZONE_DMA, and > end_zone being the idx of a ZONE_NORMAL. > > So in summary if I'm not mistaken (and I'm good at being mistaken), if we > balance pgdat and find that ZONE_NORMAL or higher needs scanning, we'll end > up trying to flush the crap out of ZONE_DMA. If we're under memory pressure, kswapd will try to free up any candidate zone, yes. > > On my test case this indeed happens and my ZONE_DMA never goes below 3000 > pages free. If I lower the reserve even further my pages free gets stuck at > 3208 and can't free any more, and doesn't ever drop below that either. > > Here is the patch I was proposing What problem does that fix though? -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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