Re: [PATCH 2/6] vhost_net: use vhost_add_used_and_signal_n() in vhost_zerocopy_signal_used()
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2013-08-25 11:47:04
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kvm, lkml, virtualization
On Fri, Aug 23, 2013 at 04:50:38PM +0800, Jason Wang wrote:
On 08/20/2013 10:33 AM, Jason Wang wrote:quoted
On 08/16/2013 05:54 PM, Michael S. Tsirkin wrote:quoted
On Fri, Aug 16, 2013 at 01:16:26PM +0800, Jason Wang wrote:quoted
quoted
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@redhat.com>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