Re: [RFC PATCH] net/virtio: Align Virtio-net header on cache line in receive path

On 03/08/2017 07:01 AM, Yao, Lei A wrote:

quoted

-----Original Message-----
From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
Sent: Monday, March 6, 2017 10:11 PM
To: Yuanhan Liu <redacted>
Cc: Liang, Cunming <redacted>; Tan, Jianfeng
[off-list ref]; dev@dpdk.org; Wang, Zhihong
[off-list ref]; Yao, Lei A [off-list ref]
Subject: Re: [RFC PATCH] net/virtio: Align Virtio-net header on cache line in
receive path



On 03/06/2017 09:46 AM, Yuanhan Liu wrote:

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On Wed, Mar 01, 2017 at 08:36:24AM +0100, Maxime Coquelin wrote:

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On 02/23/2017 06:49 AM, Yuanhan Liu wrote:

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On Wed, Feb 22, 2017 at 10:36:36AM +0100, Maxime Coquelin wrote:

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On 02/22/2017 02:37 AM, Yuanhan Liu wrote:

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On Tue, Feb 21, 2017 at 06:32:43PM +0100, Maxime Coquelin wrote:

quoted

This patch aligns the Virtio-net header on a cache-line boundary to
optimize cache utilization, as it puts the Virtio-net header (which
is always accessed) on the same cache line as the packet header.

For example with an application that forwards packets at L2 level,
a single cache-line will be accessed with this patch, instead of
two before.

I'm assuming you were testing pkt size <= (64 - hdr_size)?

No, I tested with 64 bytes packets only.

Oh, my bad, I overlooked it. While you were saying "a single cache
line", I was thinking putting the virtio net hdr and the "whole"
packet data in single cache line, which is not possible for pkt
size 64B.

quoted

I run some more tests this morning with different packet sizes,
and also with changing the mbuf size on guest side to have multi-
buffers packets:

+-------+--------+--------+-------------------------+
| Txpkt | Rxmbuf | v17.02 | v17.02 + vnet hdr align |
+-------+--------+--------+-------------------------+
|    64 |   2048 |  11.05 |                   11.78 |
|   128 |   2048 |  10.66 |                   11.48 |
|   256 |   2048 |  10.47 |                   11.21 |
|   512 |   2048 |  10.22 |                   10.88 |
|  1024 |   2048 |   7.65 |                    7.84 |
|  1500 |   2048 |   6.25 |                    6.45 |
|  2000 |   2048 |   5.31 |                    5.43 |
|  2048 |   2048 |   5.32 |                    4.25 |
|  1500 |    512 |   3.89 |                    3.98 |
|  2048 |    512 |   1.96 |                    2.02 |
+-------+--------+--------+-------------------------+

Could you share more info, say is it a PVP test? Is mergeable on?
What's the fwd mode?

No, this is not PVP benchmark, I have neither another server nor a packet
generator connected to my Haswell machine back-to-back.

This is simple micro-benchmark, vhost PMD in txonly, Virtio PMD in
rxonly. In this configuration, mergeable is ON and no offload disabled
in QEMU cmdline.

Okay, I see. So the boost, as you have stated, comes from saving two
cache line access to one. Before that, vhost write 2 cache lines,
while the virtio pmd reads 2 cache lines: one for reading the header,
another one for reading the ether header, for updating xstats (there
is no ether access in the fwd mode you tested).

quoted

That's why I would be interested in more testing on recent hardware
with PVP benchmark. Is it something that could be run in Intel lab?

I think Yao Lei could help on that? But as stated, I think it may
break the performance for bit packets. And I also won't expect big
boost even for 64B in PVP test, judging that it's only 6% boost in
micro bechmarking.

That would be great.
Note that on SandyBridge, on which I see a drop in perf with
microbenchmark, I get a 4% gain on PVP benchmark. So on recent hardware
that show a gain on microbenchmark, I'm curious of the gain with PVP
bench.

Hi, Maxime, Yuanhan

I have execute the PVP and loopback performance test on my Ivy bridge server.
OS:Ubutnu16.04
CPU: Intel(R) Xeon(R) CPU E5-2680 v2 @ 2.80GHz
Kernal:  4.4.0
gcc : 5.4.0
I use MAC forward for test.

Performance base is commit f5472703c0bdfc29c46fc4b2ca445bce3dc08c9f,
"eal: optimize aligned memcpy on x86".
I can see big performance drop on Mergeable and no-mergeable path
after apply this patch
Mergebale Path loopback test			
packet size		Performance compare	
64		 		-21.76%
128				-17.79%
260				-20.25%
520				-14.80%
1024				-9.34%
1500				-6.16%

No-mergeable  Path loopback test	
packet size			
64				-13.72%
128				-10.35%
260				-16.40%
520				-14.78%
1024				-10.48%
1500				-6.91%

Mergeable Path PVP test			
packet size		Performance compare		
64	                                               -16.33%

No-mergeable Path PVP test			
packet size			
64		                               -8.69%

Thanks Yao for the testing.
I'm surprised of the PVP results as even on SandyBridge, where I get
perf drop on micro benchmarks, I get improvement with PVP.

I'll try to reproduce some tests with Ivy Bridge, to understand what is
happening.

Cheers,
Maxime