netdev - Re: [PATCH net-next 2/2] net/mlx4_core: Disable BF when write combining is not available

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Date:	Wed, 1 Oct 2014 09:52:05 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Or Gerlitz <ogerlitz@...lanox.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Amir Vadai <amirv@...lanox.com>,
	Jack Morgenstein <jackm@....mellanox.co.il>,
	Moshe Lazer <moshel@...lanox.com>,
	Tal Alon <talal@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>
Subject: Re: [PATCH net-next 2/2] net/mlx4_core: Disable BF when write
 combining is not available

On Wed, Oct 1, 2014 at 7:54 AM, Or Gerlitz <ogerlitz@...lanox.com> wrote:
> From: Moshe Lazer <moshel@...lanox.com>
>
> In mlx4 for better latency, we write send descriptors to a write-combining
> (WC) mapped buffer instead of ringing a doorbell and having the HW fetch
> the descriptor from system memory.
>
> However, if write-combining is not supported on the host, then we
> obtain better latency by using the doorbell-ring/HW fetch mechanism.
>
> The mechanism that uses WC is called Blue-Flame (BF). BF is beneficial
> only when the system supports write combining. When the BF buffer is
> mapped as a write-combine buffer, the HCA receives data in multi-word
> bursts. However, if the BF buffer is mapped only as non-cached, the
> HCA receives data in individual dword chunks, which harms performance.
>
> Therefore, disable blueflame when write combining is not available.

curious, what numbers you're seeing:
- bf=on with wc
- bf=on without wc
- bf=off and doorbell
they will help to justify this change.
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