netdev - Re: [PATCH 11/13] net: mvneta: implement rx

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Message-Id: <20140116.114905.1582218754259744747.davem@davemloft.net>
Date:	Thu, 16 Jan 2014 11:49:05 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	w@....eu
Cc:	David.Laight@...LAB.COM, netdev@...r.kernel.org,
	thomas.petazzoni@...e-electrons.com,
	gregory.clement@...e-electrons.com
Subject: Re: [PATCH 11/13] net: mvneta: implement rx_copybreak

From: Willy Tarreau <w@....eu>
Date: Thu, 16 Jan 2014 10:36:40 +0100

> On Thu, Jan 16, 2014 at 09:14:00AM +0000, David Laight wrote:
>> From: Of Willy Tarreau
>> > calling dma_map_single()/dma_unmap_single() is quite expensive compared
>> > to copying a small packet. So let's copy short frames and keep the buffers
>> > mapped. We set the limit to 256 bytes which seems to give good results both
>> > on the XP-GP board and on the AX3/4.
>> 
>> Have you tried something similar for transmits?
>> Copying short tx into a preallocated memory area that is dma_mapped
>> at driver initialisation?
> 
> Not yet by lack of time, but I intend to do so. Both the NIC and the driver
> are capable of running at line rate (1.488 Mpps) when filling descriptors
> directly, so many improvements are still possible.

I don't know if this is relevant for your hardware, but there is a trick
with IOMMUs that I at least at one point was doing on sparc64.

I never flushed the IOMMU mappings explicitly on a dma_unmap call.

Instead I always allocate by moving the allocation cursor to higher
addresses in the IOMMU range looking for free space.

Then when I hit the end of the range and had to wrap the allocation
cursor back to the beginning, I flush the entire IOMMU.

So for %99 of IOMMU mapping creation and teardown calls, I didn't even
touch the hardware.
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