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 13:53:31 +0300
From:   Tariq Toukan <tariqt@...lanox.com>
To:     David Laight <David.Laight@...LAB.COM>,
        '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



On 20/06/2017 11:45 AM, David Laight wrote:
> 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'.
Yeah i guess my compiler does that, because it's a constant known at 
compile-time.

> 
> ...
> 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).

That sounds good!
I'll prepare and send this after testing.
I'll also look for similar cases in driver.

> 
> 	David
> 

Thank you David!