On Mon, Jun 19, 2006 at 10:27:39AM +1000, Neil Brown wrote: > On Monday June 19, dgc@xxxxxxx wrote: > > > > > I can see that shrink_dcache_sb could take a long time and should be > > > fixed, which should be as simple as replacing it with > > > shrink_dcache_parent; shrink_dcache_anon. > > > > But these are not guaranteed to reclaim all the dentries from a given > > superblock. Yes, they move the dentries to the LRU, but other activity in the > > system means that they may not get reclaimed during the subsequent calls > > to prune_dcache() and hence they may live beyond the unmount.... > > > > My proposed patch earlier in this thread (I can post it again if you > like) addresses exactly this issue. Instead of moving dentries to the > global LRU, it moves them to a private LRU, and the calls prune_dcache > on that. So there is no room for other activity to get in the way of > prune_dcache doing what needs to be done. Ok. That sounds like it would work. > > > But I'm still puzzled as to why a long dcache LRU slows down > > > unmounting. > > > > > > Can you give more details? > > > > It's not the unmount that slows down - it's the fact that the dcache lock > > is held for so long that rest of the system halts for time it takes > > to run shrink_dcache_sb(). We've seen up to 50s to do a (touch fred; rm fred) > > when the LRU has grown to several million dentries and shrink_dcache_sb() > > is running. When this happens, it's not uncommon to see every CPU in the > > machine spinning on the dcache_lock... > > Definitely a problem. > Maybe it was hoped that the call to cond_resched_lock(&dcache_lock) > would avoid this, but apparently not. The first pass over the LRU doesn't even do that.... > I still maintain that we should replace shrink_dcache_sb with calls to > shrink_dcache_anon and shrink_dcache_parent. That, together with my > previous patch, should fix this problem quite cleanly. If I send you > a combined patch against the latest -mm can you test? Ok. Send me the patch and I'll try to get some tests done on it... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - 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
