On Sat, Jun 17, 2006 at 08:25:14AM +1000, Neil Brown wrote: > On Friday June 16, jblunck@xxxxxxx wrote: > > On Mon, Jun 05, Neil Brown wrote: > > > > > I understand that this is where problem is because the selected > > > dentries don't stay at the end of the list very long in some > > > circumstances. In particular, other filesystems' dentries get mixed > > > in. > > > > No. The problem is that the LRU list is too long and therefore unmounting > > seems to take ages. > > > > But I cannot see that the whole LRU list needs to be scanned during > unmount. ... > 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.... > 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... 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
