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Date: Sat, 26 Nov 2016 00:17:20 +0300
From: Oleg Bulyzhin <oleg@FreeBSD.org>
To: Fabien Thomas <fabient@FreeBSD.org>
Cc: src-committers@freebsd.org, svn-src-all@freebsd.org,
 svn-src-head@freebsd.org
Subject: Re: svn commit: r309143 - head/sys/libkern
Message-ID: <20161125211720.GA66671@lath.rinet.ru>
References: <201611251349.uAPDnX09042110@repo.freebsd.org>
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On Fri, Nov 25, 2016 at 01:49:33PM +0000, Fabien Thomas wrote:

> +	if (reseed || atomic_cmpset_int(&arc4rand_iniseed_state,
> +			ARC4_ENTR_HAVE, ARC4_ENTR_SEED)) {
> +		ARC4_FOREACH(arc4)
> +			arc4_randomstir(arc4);
> +	}

How about eliminating this atomic? Converting it to:

if (reseed || arc4rand_iniseed_state == ARC4_ENTR_HAVE) {
	arc4rand_iniseed_state = ARC4_ENTR_SEED;
	ARC4_FOREACH(arc4);
		arc4_randomstir(arc4);
}
should give you free performance boost. Having locked cmpxchg in main path is
not cheap (and as i can see it's really used only once,
when randomdev unblocks).

P.S. quick and dirty test gives me 10-30% of extra perfomance.

-- 
Oleg.

================================================================
=== Oleg Bulyzhin -- OBUL-RIPN -- OBUL-RIPE -- oleg@rinet.ru ===
================================================================