On Mon, Oct 10, 2016 at 04:54:39PM +0200, Maxime Coquelin wrote:
On 10/10/2016 04:42 PM, Yuanhan Liu wrote:
On Mon, Oct 10, 2016 at 02:40:44PM +0200, Maxime Coquelin wrote:
At that time, a packet always use 2 descs. Since indirect desc is
enabled (by default) now, the assumption is not true then. What's
worse, it might even slow things a bit down. That should also be
part of the reason why performance is slightly worse than before.
--yliu
I'm not sure I get what you are saying
commit 1d41d77cf81c448c1b09e1e859bfd300e2054a98
Author: Yuanhan Liu <address@hidden>
Date: Mon May 2 17:46:17 2016 -0700
vhost: optimize dequeue for small packets
A virtio driver normally uses at least 2 desc buffers for Tx: the
first for storing the header, and the others for storing the data.
Therefore, we could fetch the first data desc buf before the main
loop, and do the copy first before the check of "are we done yet?".
This could save one check for small packets that just have one data
desc buffer and need one mbuf to store it.
Signed-off-by: Yuanhan Liu <address@hidden>
Acked-by: Huawei Xie <address@hidden>
Tested-by: Rich Lane <address@hidden>
This fast-paths the 2-descriptors format but it's not active
for indirect descriptors. Is this what you mean?
Yes. It's also not active when ANY_LAYOUT is actually turned on.
Should be a simple matter to apply this optimization for indirect.
Might be.
If I understand the code correctly, indirect descs also benefit from this
optimization, or am I missing something?
Aha..., you are right!
The interesting thing is that the patch I send on Thursday that removes
header access when no offload has been negotiated[0] seems to reduce
almost to zero the performance seen with indirect descriptors enabled.
Didn't follow that.
I see this with 64 bytes packets using testpmd on both ends.
When I did the patch, I would have expected the same gain with both
modes, whereas I measured +1% for direct and +4% for indirect.
IIRC, I did a test before (remove those offload code piece), and the
performance was basically the same before and after that. Well, there
might be some small difference, say 1% as you said. But the result has
never been steady.
Anyway, I think your patch is good to have: I just didn't see v2.