Messages in this thread |  | | Subject | Re: [PATCH net-next V2 1/3] tap: use build_skb() for small packet | From | Jason Wang <> | Date | Wed, 16 Aug 2017 12:07:25 +0800 |
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On 2017年08月16日 11:59, Michael S. Tsirkin wrote: > On Wed, Aug 16, 2017 at 11:57:51AM +0800, Jason Wang wrote: >> >> On 2017年08月16日 11:55, Michael S. Tsirkin wrote: >>> On Tue, Aug 15, 2017 at 08:45:20PM -0700, Eric Dumazet wrote: >>>> On Fri, 2017-08-11 at 19:41 +0800, Jason Wang wrote: >>>>> We use tun_alloc_skb() which calls sock_alloc_send_pskb() to allocate >>>>> skb in the past. This socket based method is not suitable for high >>>>> speed userspace like virtualization which usually: >>>>> >>>>> - ignore sk_sndbuf (INT_MAX) and expect to receive the packet as fast as >>>>> possible >>>>> - don't want to be block at sendmsg() >>>>> >>>>> To eliminate the above overheads, this patch tries to use build_skb() >>>>> for small packet. We will do this only when the following conditions >>>>> are all met: >>>>> >>>>> - TAP instead of TUN >>>>> - sk_sndbuf is INT_MAX >>>>> - caller don't want to be blocked >>>>> - zerocopy is not used >>>>> - packet size is smaller enough to use build_skb() >>>>> >>>>> Pktgen from guest to host shows ~11% improvement for rx pps of tap: >>>>> >>>>> Before: ~1.70Mpps >>>>> After : ~1.88Mpps >>>>> >>>>> What's more important, this makes it possible to implement XDP for tap >>>>> before creating skbs. >>>> Well well well. >>>> >>>> You do realize that tun_build_skb() is not thread safe ? >>> The issue is alloc frag, isn't it? >>> I guess for now we can limit this to XDP mode only, and >>> just allocate full pages in that mode. >>> >>> >> Limit this to XDP mode only does not prevent user from sending packets to >> same queue in parallel I think? >> >> Thanks > Yes but then you can just drop the page frag allocator since > XDP is assumed not to care about truesize for most packets. >
Ok, let me do some test to see the numbers between the two methods first.
Thanks
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