On Wed, Nov 21, 2012 at 02:07:04PM +0100, Michal Nazarewicz wrote: > On Wed, Nov 21 2012, Minchan Kim wrote: > > So your concern is that too many free pages in MIGRATE_CMA when OOM happens > > is odd? It's natural with considering CMA design which kernel never fallback > > non-movable page allocation to CMA area. I guess it's not a your concern. > > > > Let's think below extreme cases. > > > > = Before = > > > > * 1000M DRAM system. > > * 400M kernel used pages. > > * 300M movable used pages. > > * 300M cma freed pages. > > > > 1. kernel want to request 400M non-movable memory, additionally. > > 2. VM start to reclaim 300M movable pages. > > 3. But it's not enough to meet 400M request. > > 4. go to OOM. (It's natural) > > > > = After(with your patch) = > > > > * 1000M DRAM system. > > * 400M kernel used pages. > > * 300M movable *freed* pages. > > * 300M cma used pages(by your patch, I simplified your concept) > > > > 1. kernel want to request 400M non-movable memory. > > 2. 300M movable freed pages isn't enough to meet 400M request. > > 3. Also, there is no point to reclaim CMA pages for non-movable allocation. > > 4. go to OOM. (It's natural) > > > > There is no difference between before and after in allocation POV. > > Let's think another example. > > > > = Before = > > > > * 1000M DRAM system. > > * 400M kernel used pages. > > * 300M movable used pages. > > * 300M cma freed pages. > > > > 1. kernel want to request 300M non-movable memory. > > 2. VM start to reclaim 300M movable pages. > > 3. It's enough to meet 300M request. > > 4. happy end > > > > = After(with your patch) = > > > > * 1000M DRAM system. > > * 400M kernel used pages. > > * 300M movable *freed* pages. > > * 300M cma used pages(by your patch, I simplified your concept) > > > > 1. kernel want to request 300M non-movable memory. > > 2. 300M movable freed pages is enough to meet 300M request. > > 3. happy end. > > > > There is no difference in allocation POV, too. > > The difference thou is that before 30% of memory is wasted (ie. free), > whereas after all memory is used. The main point of CMA is to make the > memory useful if devices are not using it. Having it not allocated is > defeating that purpose.
I think it's not a waste because if reclaimed movable pages is working set, they are soon reloaded to migrate_cma in this time. > > -- > Best regards, _ _ > .o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o > ..o | Computer Science, Michał “mina86” Nazarewicz (o o) > ooo +----<email/xmpp: m...@google.com>--------------ooO--(_)--Ooo-- -- Kind Regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
