svn commit: r309143 - head/sys/libkern

[Mateusz Guzik]

On Fri, Nov 25, 2016 at 01:49:33PM +0000, Fabien Thomas wrote:
> Author: fabient
> Date: Fri Nov 25 13:49:33 2016
> New Revision: 309143
> URL: https://svnweb.freebsd.org/changeset/base/309143
> 
> Log:
>   In a dual processor system (2*6 cores) during IPSec throughput tests,
>   we see a lot of contention on the arc4 lock, used to generate the IV
>   of the ESP output packets.
>   
>   The idea of this patch is to split this mutex in order to reduce the
>   contention on this lock.
>   
> +MALLOC_DEFINE(M_ARC4RANDOM, "arc4random", "arc4random structures");
>  
> -static u_int8_t arc4_randbyte(void);
> +struct arc4_s {
> +	u_int8_t i, j;
> +	int numruns;
> +	u_int8_t sbox[256];
> +	time_t t_reseed;
> +
> +	struct mtx mtx;
> +};
> +

Why is the lock at the end? Right now you got false sharing with the
next entry.

That said, I suggest:
1. moving thelock to the beginning
2. annotating the struct with __aligned(CACHE_LINE_SIZE)

-- 
Mateusz Guzik <mjguzik gmail.com>