Re: [PATCH v2 0/5] virtio: Tx performance improvements

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 22 Oct 2015 10:38:33 +0000
"Xie, Huawei" <huawei.xie@intel.com> wrote:

> On 10/21/2015 9:20 PM, Thomas Monjalon wrote:
> > 2015-10-18 22:16, Stephen Hemminger:
> >> This is a tested version of the virtio Tx performance improvements
> >> that I posted earlier on the list, and described at the DPDK Userspace
> >> meeting in Dublin. Together they get a 25% performance improvement for
> >> both small packet and large multi-segment packet case when testing
> >> from DPDK guest application to Linux KVM host.
> >>
> >> Stephen Hemminger (5):
> >>   virtio: clean up space checks on xmit
> >>   virtio: don't use unlikely for normal tx stuff
> >>   virtio: use indirect ring elements
> >>   virtio: use any layout on transmit
> >>   virtio: optimize transmit enqueue
> > Huawei, do you ack this series?
> >
> Okay with this patchset with two remained questions,
> 
> +/* Region reserved to allow for transmit header and indirect ring */
> +#define VIRTIO_MAX_TX_INDIRECT 8
> +struct virtio_tx_region {
> +	struct virtio_net_hdr_mrg_rxbuf tx_hdr;
> 
> Why use merge-able rx header here in the tx region?

If mergeable rx is negotiated then the header must be used for
both Tx and Rx. I chose to allocate the largest possible header
needed, rather than having to deal with variable size data structure.

> 
> > +	struct vring_desc tx_indir[VIRTIO_MAX_TX_INDIRECT]
> > +			   __attribute__((__aligned__(16)));
> 
> WARNING: __aligned(size) is preferred over __attribute__((aligned(size)))

That is true in kernel, but there is no __aligned macro in DPDK code.
It could be changed to __rte_aligned_16?