On Mon, Feb 23, 2015 at 05:32:52PM -0500, David Miller wrote: > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> > Date: Mon, 23 Feb 2015 13:52:49 -0800 > > > On Mon, Feb 23, 2015 at 09:03:58PM +0000, Thomas Graf wrote: > >> On 02/23/15 at 11:12am, josh@xxxxxxxxxxxxxxxx wrote: > >> > In theory, resizes should only take the locks for the buckets they're > >> > currently unzipping, and adds should take those same locks. Neither one > >> > should take a whole-table lock, other than resize excluding concurrent > >> > resizes. Is that still insufficient? > >> > >> Correct, this is what happens. The problem is basically that > >> if we insert from atomic context we cannot slow down inserts > >> and the table may not grow quickly enough. > >> > >> > Yeah, the add/remove statistics used for tracking would need some > >> > special handling to avoid being a table-wide bottleneck. > >> > >> Daniel is working on a patch to do per-cpu element counting > >> with a batched update cycle. > > > > One approach is simply to count only when a resize operation is in > > flight. Another is to keep a per-bucket count, which can be summed > > at the beginning of the next resize operation. > > I think we should think exactly about what we should do when someone > loops non-stop adding 1 million entries to the hash table and the > initial table size is very small. > > This is a common use case for at least one of the current rhashtable > users (nft_hash). When you load an nftables rule with a large set > of IP addresses attached, this is what happens. > > Yes I understand that nftables could give a hint and start with a > larger hash size from the start when it knows this is going to happen, > but I still believe that we should behave reasonably when starting > from a small table. > > I'd say that with the way things work right now, in this situation it > actually hurts to allow asynchronous inserts during a resize. Because > we end up with extremely long hash table chains, and thus make the > resize work and the lookups both take an excruciatingly long amount of > time to complete. > > I just did a quick scan of all code paths that do inserts into an > rhashtable, and it seems like all of them can easily block. So why > don't we do that? Make inserts sleep on an rhashtable expansion > waitq. > > There could even be a counter of pending inserts, so the expander can > decide to expand further before waking the inserting threads up. Should be reasonably simple, and certainly seems worth a try! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
