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 <yuanhan.liu at linux.intel.com> >>>>> 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 <yuanhan.liu at linux.intel.com> >>>>> Acked-by: Huawei Xie <huawei.xie at intel.com> >>>>> Tested-by: Rich Lane <rich.lane at bigswitch.com> >>>> >>>> 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. 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. Maxime [0]: http://dpdk.org/dev/patchwork/patch/16423/
