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

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Date:	Thu, 26 Feb 2015 07:53:55 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...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 25.02, Alexei Starovoitov wrote:
> On Wed, Feb 25, 2015 at 12:10 PM, Patrick McHardy <kaber@...sh.net> wrote:
> > On 25.02, Eric Dumazet wrote:
> >> But if any workload had to grow the table to 2^20 slots, we had to
> >> consume GB of memory anyway to hold sockets and everything.
> >>
> >> Trying to shrink is simply not worth it, unless you expect your host
> >> never reboots and you desperately need back these 8 MBytes of memory.
> >
> > That may be true in the TCP case, but for not for nftables. We might
> > have many sets and, especially when used to represent more complicated
> > classification algorithms, their size might change by a lot.
> 
> sounds like grow/shrink decision cannot be generalized within
> rhashtable, but two callbacks are about to be removed and the
> are costly. So would it make sense to disable auto-expand/shrink
> completely and let nft/tcp call expand/shrink when needed?

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?

> nft can potentially do smarter batching this way.
> If it sees a lot of entries are about to be inserted, it can call
> expand directly to quickly grow sparsely populated table
> into large one, and then insert all the entries.
> That will mitigate 'slow rcu' issue as well.

I like that idea.
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