netdev - Re: Ottawa and slow hash-table resize

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Message-ID: <20150223215248.GA15405@linux.vnet.ibm.com>
Date:	Mon, 23 Feb 2015 13:52:49 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	josh@...htriplett.org, alexei.starovoitov@...il.com,
	herbert@...dor.apana.org.au, kaber@...sh.net, davem@...emloft.net,
	ying.xue@...driver.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Ottawa and slow hash-table resize

On Mon, Feb 23, 2015 at 09:03:58PM +0000, Thomas Graf wrote:
> On 02/23/15 at 11:12am, josh@...htriplett.org 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.

							Thanx, Paul

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