netdev - Re: what is the expeted performance from a dual port 10G card ?

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Message-ID: <5d96567b0811292325j39f39a9ew343b430de72453d4@mail.gmail.com>
Date:	Sun, 30 Nov 2008 09:25:55 +0200
From:	Raz <raziebe@...il.com>
To:	"Trent Piepho" <tpiepho@...escale.com>
Cc:	"Roland Dreier" <rdreier@...co.com>,
	"Ben Hutchings" <bhutchings@...arflare.com>, netdev@...r.kernel.org
Subject: Re: what is the expeted performance from a dual port 10G card ?

On Sun, Nov 30, 2008 at 4:55 AM, Trent Piepho <tpiepho@...escale.com> wrote:
> On Sat, 29 Nov 2008, Roland Dreier wrote:
>> > The 8b10 encoding is already accounted for in the 2.5 Gbps figure;
>> > the raw bit rate is 3.125 Gbps.
>>
>> In addition, PCI Express transfers are broken up into packets, usually
>> with very small payloads (128 or 256 bytes are common).  So the packet
>> header overhead reduces throughput further, and then the transaction
>> layer adds further overhead.  Then transferring NIC control structures
>> over the link adds even more overhead.  So achieving 13.something Gb/sec
>> of real throughput on a PCIe link theoretically capable of 16 Gb/sec
>> seems pretty good to me.
I tested it both on PCIE2 and PCIE1 . same results.

> For transfers with only 4 byte payloads, I get about 6.5 MB/sec over a PCIe
> 4x link.  For small transfers, the overhead is huge.
>
Well, you are correct. I used packet size of 1.3k. when I increased to
9K a packet each port
reached ~7Gbps ( I am using pktgen). I used two cards, so I reached
28Gbps. BIOS settings for pci payload were AUTO,so i set it to 256
bytes but it made no difference, still 28Gbps. well, at least I know
what the problem is.
guys, I truly thank you all.
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