On Fri, Aug 23, 2013 at 04:50:38PM +0800, Jason Wang wrote: > On 08/20/2013 10:33 AM, Jason Wang wrote: > > On 08/16/2013 05:54 PM, Michael S. Tsirkin wrote: > >> On Fri, Aug 16, 2013 at 01:16:26PM +0800, Jason Wang wrote: > >>>> Switch to use vhost_add_used_and_signal_n() to avoid multiple calls to > >>>> vhost_add_used_and_signal(). With the patch we will call at most 2 times > >>>> (consider done_idx warp around) compared to N times w/o this patch. > >>>> > >>>> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx> > >> So? Does this help performance then? > >> > > Looks like it can especially when guest does support event index. When > > guest enable tx interrupt, this can saves us some unnecessary signal to > > guest. I will do some test. > > Have done some test. I can see 2% - 3% increasing in both aggregate > transaction rate and per cpu transaction rate in TCP_RR and UDP_RR test. > > I'm using ixgbe. W/o this patch, I can see more than 100 calls of > vhost_add_used_signal() in one vhost_zerocopy_signaled_used(). This is > because ixgbe (and other modern ethernet driver) tends to free old tx > skbs in a loop during tx interrupt, and vhost tend to batch the adding > used and signal in vhost_zerocopy_callback(). Switching to use > vhost_add_use_and_signal_n() means saving 100 times of used idx updating > and memory barriers. Well it's only smp_wmb so a nop on most architectures, so a 2% gain is surprising. I'm guessing the cache miss on the write is what's giving us a speedup here. I'll review the code, thanks. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
