netdev - Re: [PATCH net 2/2] rhashtable: remove indirection for grow/shrink decision functions

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Date:	Thu, 26 Feb 2015 06:50:36 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	David Laight <David.Laight@...lab.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"tgraf@...g.ch" <tgraf@...g.ch>,
	"pablo@...filter.org" <pablo@...filter.org>,
	"johunt@...mai.com" <johunt@...mai.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net 2/2] rhashtable: remove indirection for grow/shrink
 decision functions

On Thu, 2015-02-26 at 07:53 +0000, Patrick McHardy wrote:

> My understanding was that Eric was arguing against shrinking in general.
> But assuming we have it, what's the downside of also performing
> shrinking for TCP?

Hash resize is horribly expensive (this is general to all hash
implementations), so once you took the risk to expand the table once,
you do not want to take the risk another time. You are lucky if host was
not already taken out of the server pool.

Fact that cond_resched() is not yet used anywhere in lib/rhashtable.c
reveals part of the problem we have here.

(I'll submit patches for that)


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