linux-kernel - Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20061204.135625.48528445.davem@davemloft.net>
Date:	Mon, 04 Dec 2006 13:56:25 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	dada1@...mosbay.com
Cc:	akpm@...l.org, clameter@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] SLAB : use a multiply instead of a divide in
 obj_to_index()

From: Eric Dumazet <dada1@...mosbay.com>
Date: Mon, 04 Dec 2006 22:34:29 +0100

> On a 200 MHz sparcv9 machine, the division takes 64 cycles instead of 1 cycle
> for a multiply.

For UltraSPARC I and II (which is what this 200mhz guy probably is),
it's 4 cycle latency for a multiply (32-bit or 64-bit) and 68 cycles
for a 64-bit divide (32-bit divide is 37 cycles).

UltraSPARC-III and IV are worse, 6 cycles for multiply and 40/71
cycles (32/64-bit) for integer divides.

Niagara is even worse :-)  11 cycle integer multiply and a 72 cycle
integer divide (regardless of 32-bit or 64-bit).

(more details in gcc/config/sparc/sparc.c:{ultrasparc,ultrasparc3,niagara}_cost).

So this change has tons of merit for sparc64 chips at least :-)

Also, the multiply can parallelize with other operations but it
seems that integer divide stalls the pipe for most of the duration
of the calculation.  So this makes the divide even worse.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/