On Fri, Jul 30, 2010 at 01:54:53PM +0900, KOSAKI Motohiro wrote: > > > (1) and (8) might be solved > > > by sleeping awhile, but it's unrelated on io-congestion. but might not be. It only works > > > by lucky. So I don't like to depned on luck. > > > > In this case, waiting a while really in the right thing to do. It stalls > > the caller, but it's a high-order allocation. The alternative is for it > > to keep scanning which when under memory pressure could result in far > > too many pages being evicted. How long to wait is a tricky one to answer > > but I would recommend making this a low priority. > > For case (1), just lock_page() instead trylock is brilliant way than random sleep. > Is there any good reason to give up synchrounous lumpy reclaim when trylock_page() failed? > IOW, briefly lock_page() and wait_on_page_writeback() have the same latency. why should > we only avoid former? > No reason. Using lock_page() in the synchronous case would be a sensible choice. As you are realising, there are a number of warts around lumpy reclaim that are long overdue for a good look :/ > side note: page lock contention is very common case. > > For case (8), I don't think sleeping is right way. get_page() is used in really various place of > our kernel. so we can't assume it's only temporary reference count increasing. In what case is a munlocked pages reference count permanently increased and why is this not a memory leak? > In the other > hand, this contention is not so common because shrink_page_list() is excluded from IO > activity by page-lock and wait_on_page_writeback(). so I think giving up this case don't > makes too many pages eviction. > If you disagree, can you please explain your expected bad scinario? > Right now, I can't think of a problem with calling lock_page instead of trylock for synchronous lumpy reclaim. > > > > > 3. pageout() is intended anynchronous api. but doesn't works so. > > > > > > > > > > pageout() call ->writepage with wbc->nonblocking=1. because if the system have > > > > > default vm.dirty_ratio (i.e. 20), we have 80% clean memory. so, getting stuck > > > > > on one page is stupid, we should scan much pages as soon as possible. > > > > > > > > > > HOWEVER, block layer ignore this argument. if slow usb memory device connect > > > > > to the system, ->writepage() will sleep long time. because submit_bio() call > > > > > get_request_wait() unconditionally and it doesn't have any PF_MEMALLOC task > > > > > bonus. > > > > > > > > Is this not a problem in the writeback layer rather than pageout() > > > > specifically? > > > > > > Well, outside pageout(), probably only XFS makes PF_MEMALLOC + writeout. > > > because PF_MEMALLOC is enabled only very limited situation. but I don't know > > > XFS detail at all. I can't tell this area... > > > > > > > All direct reclaimers have PF_MEMALLOC set so it's not that limited a > > situation. See here > > Yes, all direct reclaimers have PF_MEMALLOC. but usually all direct reclaimers don't call > any IO related function except pageout(). As far as I know, current shrink_icache() and > shrink_dcache() doesn't make IO. Am I missing something? > Not that I'm aware of but it's not something I would know offhand. Will go digging. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
