netdev - Re: [PATCH net] x86: bpf_jit: fix compilation of large bpf programs

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Message-ID: <1432650922.4060.236.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 26 May 2015 07:35:22 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	'Alexei Starovoitov' <ast@...mgrid.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>,
	Eric Dumazet <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] x86: bpf_jit: fix compilation of large bpf programs

On Tue, 2015-05-26 at 13:40 +0000, David Laight wrote:

> If the JIT compiler is only changing the encoding of the constants
> in the x86 instructions (rather than changing the instructions themselves)
> then there is likely to me an unmeasurable change in the execution time.
> For instance I don't remember there being a difference in execution time
> between long and short branches - the only difference is the amount of
> cache they use.

icache is precisely the matter here. In the end, it makes a difference.

You could check this interesting study Ingo did recently :

https://lkml.org/lkml/2015/5/19/1009


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