On Fri, 2010-08-27 at 05:30 +0800, Andrew Morton wrote: > On Thu, 26 Aug 2010 15:59:10 +0800 > Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > > > On Thu, Aug 26, 2010 at 04:03:18AM +0800, Andrew Morton wrote: > > > On Fri, 6 Aug 2010 11:08:05 +0800 > > > Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > > > > > > > Subject: mm: batch activate_page() to reduce lock contention > > > > > ... > > > > > This function is pretty bizarre. It really really needs some comments > > > explaining what it's doing and most especially *why* it's doing it. > > > > > > It's a potential O(n*nr_zones) search (I think)! We demand proof that > > > it's worthwhile! > > > > > > Yes, if the pagevec is filled with pages from different zones then it > > > will reduce the locking frequency. But in the common case where the > > > pagevec has pages all from the same zone, or has contiguous runs of > > > pages from different zones then all that extra bitmap fiddling gained > > > us nothing. > > > > > > (I think the search could be made more efficient by advancing `i' when > > > we first see last_zone!=page_zone(page), but that'd just make the code > > > even worse). > > Thanks for pointing this out. Then we can simplify things a little bit. > > the 144 bytes footprint is because of this too, then we can remove it. > > ok.. > > > > > > > There's a downside/risk to this code. A billion years ago I found > > > that it was pretty important that if we're going to batch pages in this > > > manner, it's important that ALL pages be batched via the same means. > > > If 99% of the pages go through the pagevec and 1% of pages bypass the > > > pagevec, the LRU order gets scrambled and we can end up causing > > > additional disk seeks when the time comes to write things out. The > > > effect was measurable. > > > > > > And lo, putback_lru_pages() (at least) bypasses your new pagevecs, > > > potentially scrambling the LRU ordering. Admittedly, if we're putting > > > back unreclaimable pages in there, the LRU is probably already pretty > > > scrambled. But that's just a guess. > > ok, we can drain the pagevecs in putback_lru_pages() or add active page > > to the new pagevecs. > > The latter I guess? hi, looks the lru_add_pvecs pagevecs is bypassed too in putback_lru_pages(). Assume the bypass doesn't has obvious impact? each pagevec stores 14 pages, it should be < 1/1000 total memory in typical systems. so I wonder if we really need handle the active page pagevecs bypass. Thanks, Shaohua -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>