netdev - RE: [PATCH net-next 09/10] net/mlx4_en: Replace TXBB_SIZE multiplications with shift operations

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Date:   Tue, 20 Jun 2017 08:45:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Tariq Toukan' <tariqt@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "kernel-team@...com" <kernel-team@...com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH net-next 09/10] net/mlx4_en: Replace TXBB_SIZE
 multiplications with shift operations

From: Tariq Toukan
> Sent: 15 June 2017 12:36
> Define LOG_TXBB_SIZE, log of TXBB_SIZE, and use it with a shift
> operation instead of a multiplication with TXBB_SIZE.
> Operations are equivalent as TXBB_SIZE is a power of two.
> 
> Performance tests:
> Tested on ConnectX3Pro, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
> 
> Gain is too small to be measurable, no degradation sensed.
> Results are similar for IPv4 and IPv6.

I can't imagine there is any difference at all.
The compiler will use a shift for a 'multiply by a constant power of 2'.

...
If you want to save a few cycles I think the loop:
> -		for (i = 0; i < tx_info->nr_txbb * TXBB_SIZE;
> +		for (i = 0; i < tx_info->nr_txbb << LOG_TXBB_SIZE;
requires the compiler generate code to read nr_txbb every
iteration.
Caching the values might help (unless it causes a different
register spill).

	David