Messages in this thread | | | Date | Tue, 9 Jun 2009 16:45:50 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 2/3] Properly account for the number of page cache pages zone_reclaim() can reclaim |
| |
On Tue, Jun 09, 2009 at 04:27:29PM +0800, Mel Gorman wrote: > On Tue, Jun 09, 2009 at 10:25:49AM +0800, Wu Fengguang wrote: > > On Mon, Jun 08, 2009 at 09:01:29PM +0800, Mel Gorman wrote: > > > On NUMA machines, the administrator can configure zone_relcaim_mode that > > > is a more targetted form of direct reclaim. On machines with large NUMA > > > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean > > > unmapped pages will be reclaimed if the zone watermarks are not being met. > > > > > > There is a heuristic that determines if the scan is worthwhile but the > > > problem is that the heuristic is not being properly applied and is basically > > > assuming zone_reclaim_mode is 1 if it is enabled. > > > > > > This patch makes zone_reclaim() makes a better attempt at working out how > > > many pages it might be able to reclaim given the current reclaim_mode. If it > > > cannot clean pages, then NR_FILE_DIRTY number of pages are not candidates. If > > > it cannot swap, then NR_FILE_MAPPED are not. This indirectly addresses tmpfs > > > as those pages tend to be dirty as they are not cleaned by pdflush or sync. > > > > No, tmpfs pages are not accounted in NR_FILE_DIRTY because of the > > BDI_CAP_NO_ACCT_AND_WRITEBACK bits. > > > > Ok, that explains why the dirty page count was not as high as I was > expecting. Thanks. > > > > The ideal would be that the number of tmpfs pages would also be known > > > and account for like NR_FILE_MAPPED as swap is required to discard them. > > > A means of working this out quickly was not obvious but a comment is added > > > noting the problem. > > > > I'd rather prefer it be accounted separately than to muck up NR_FILE_MAPPED :) > > > > Maybe I used a poor choice of words. What I meant was that the ideal would > be we had a separate count for tmpfs pages. As tmpfs pages and mapped pages > both have to be unmapped and potentially, they are "like" each other with > respect to the zone_reclaim_mode and how it behaves. We would end up > with something like > > pagecache_reclaimable -= zone_page_state(zone, NR_FILE_MAPPED); > pagecache_reclaimable -= zone_page_state(zone, NR_FILE_TMPFS);
OK. But tmpfs pages may be mapped, so there will be double counting. We must at least make sure pagecache_reclaimable won't get underflowed. (Or make another LRU list for tmpfs pages?)
> > > + int pagecache_reclaimable; > > > + > > > + /* > > > + * Work out how many page cache pages we can reclaim in this mode. > > > + * > > > + * NOTE: Ideally, tmpfs pages would be accounted as if they were > > > + * NR_FILE_MAPPED as swap is required to discard those > > > + * pages even when they are clean. However, there is no > > > + * way of quickly identifying the number of tmpfs pages > > > + */ > > > > So can you remove the note on NR_FILE_MAPPED? > > > > Why would I remove the note? I can alter the wording but the intention is > to show we cannot count the number of tmpfs pages quickly and it would be > nice if we could. Maybe this is clearer? > > Note: Ideally tmpfs pages would be accounted for as NR_FILE_TMPFS or > similar and treated similar to NR_FILE_MAPPED as both require > unmapping from page tables and potentially swap to reclaim. > However, no such counter exists.
That's better. Thanks.
> > > + pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES); > > > + if (!(zone_reclaim_mode & RECLAIM_WRITE)) > > > + pagecache_reclaimable -= zone_page_state(zone, NR_FILE_DIRTY); > > > > > + if (!(zone_reclaim_mode & RECLAIM_SWAP)) > > > + pagecache_reclaimable -= zone_page_state(zone, NR_FILE_MAPPED); > > > > So the "if" can be removed because NR_FILE_MAPPED is not related to swapping? > > > > It's partially related with respect to what zone_reclaim() is doing. > Once something is mapped, we need RECLAIM_SWAP set on the > zone_reclaim_mode to do anything useful with them.
You are referring to mapped anonymous/tmpfs pages? But I mean NR_FILE_MAPPED pages won't goto swap when unmapped.
Thanks, Fengguang
> > > /* > > > * Zone reclaim reclaims unmapped file backed pages and > > > @@ -2391,8 +2406,7 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order) > > > * if less than a specified percentage of the zone is used by > > > * unmapped file backed pages. > > > */ > > > - if (zone_page_state(zone, NR_FILE_PAGES) - > > > - zone_page_state(zone, NR_FILE_MAPPED) <= zone->min_unmapped_pages > > > + if (pagecache_reclaimable <= zone->min_unmapped_pages > > > && zone_page_state(zone, NR_SLAB_RECLAIMABLE) > > > <= zone->min_slab_pages) > > > return 0; > > > -- > > > 1.5.6.5 > > > > -- > Mel Gorman > Part-time Phd Student Linux Technology Center > University of Limerick IBM Dublin Software Lab
| |