netdev - Re: Ottawa and slow hash-table resize

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Date:	Tue, 24 Feb 2015 10:45:02 -0800
From:	josh@...htriplett.org
To:	David Miller <davem@...emloft.net>
Cc:	tgraf@...g.ch, kaber@...sh.net, paulmck@...ux.vnet.ibm.com,
	alexei.starovoitov@...il.com, herbert@...dor.apana.org.au,
	ying.xue@...driver.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Ottawa and slow hash-table resize

On Tue, Feb 24, 2015 at 01:26:03PM -0500, David Miller wrote:
> From: Thomas Graf <tgraf@...g.ch>
> Date: Tue, 24 Feb 2015 17:50:14 +0000
> 
> > On 02/24/15 at 12:09pm, David Miller wrote:
> >> Thinking about this, if inserts occur during a pending resize, if the
> >> nelems of the table has exceeded even the grow threshold for the new
> >> table, it makes no sense to allow these async inserts as they are
> >> going to make the resize take longer and prolong the pain.
> > 
> > Let's say we start with an initial table size of 16K (we can make
> > this system memory depenend) and we grow by 8x. New inserts go
> > into the new table immediately so as soon as we have 12K entries
> > we'll grow right to 128K buckets. As we grow above 75K we'll start
> > growing to 1024K buckets. New entries already go to the 1024K
> > buckets at this point given that the first grow cycle should be
> > fast. The 2nd grow cycle would take an est 6 RCU grace periods.
> > This would also still give us a max of 8K bucket locks which
> > should be good enough as well.
> 
> Actually, first of all, let's not start with larger tables.
> 
> The network namespace folks showed clearly that hash tables
> are detrimental to per-ns memory costs.  So they definitely
> want us to start with extremely small tables.

Agreed; ideally, the initial table size would just use a single page for
the array of bucket heads, which would give 1024 buckets on 32-bit
systems or 512 on 64-bit systems.  That's more than enough for many
client systems, and for many single-application network namespaces.

> But once we know something is actively used, sure, increase
> the table grow rate as a response to demand.
> 
> So how feasible is it to grow by 4x, 8x, or other powers of
> two in one resize operation?

Quite feasible.  Actually, any integer multiple works fine, though I
think a power of two makes sense.  I'd suggest trying 4x with the same
workloads that had an issue at 2x, and seeing how that goes.

- Josh Triplett
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