netdev - Re: Ottawa and slow hash-table resize

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Date:	Tue, 24 Feb 2015 13:26:03 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	tgraf@...g.ch
Cc:	kaber@...sh.net, paulmck@...ux.vnet.ibm.com, josh@...htriplett.org,
	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

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.

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?
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