On 10/15/2014 05:51 AM, Michael S. Tsirkin wrote: > On Tue, Oct 14, 2014 at 02:53:27PM -0400, David Miller wrote: >> From: Jason Wang <jasowang@xxxxxxxxxx> >> Date: Sat, 11 Oct 2014 15:16:43 +0800 >> >>> We free old transmitted packets in ndo_start_xmit() currently, so any >>> packet must be orphaned also there. This was used to reduce the overhead of >>> tx interrupt to achieve better performance. But this may not work for some >>> protocols such as TCP stream. TCP depends on the value of sk_wmem_alloc to >>> implement various optimization for small packets stream such as TCP small >>> queue and auto corking. But orphaning packets early in ndo_start_xmit() >>> disable such things more or less since sk_wmem_alloc was not accurate. This >>> lead extra low throughput for TCP stream of small writes. >>> >>> This series tries to solve this issue by enable tx interrupts for all TCP >>> packets other than the ones with push bit or pure ACK. This is done through >>> the support of urgent descriptor which can force an interrupt for a >>> specified packet. If tx interrupt was enabled for a packet, there's no need >>> to orphan it in ndo_start_xmit(), we can free it tx napi which is scheduled >>> by tx interrupt. Then sk_wmem_alloc was more accurate than before and TCP >>> can batch more for small write. More larger skb was produced by TCP in this >>> case to improve both throughput and cpu utilization. >>> >>> Test shows great improvements on small write tcp streams. For most of the >>> other cases, the throughput and cpu utilization are the same in the >>> past. Only few cases, more cpu utilization was noticed which needs more >>> investigation. >>> >>> Review and comments are welcomed. >> I think proper accounting and queueing (at all levels, not just TCP >> sockets) is more important than trying to skim a bunch of cycles by >> avoiding TX interrupts. >> >> Having an event to free the SKB is absolutely essential for the stack >> to operate correctly. >> >> And with virtio-net you don't even have the excuse of "the HW >> unfortunately doesn't have an appropriate TX event." >> >> So please don't play games, and instead use TX interrupts all the >> time. You can mitigate them in various ways, but don't turn them on >> selectively based upon traffic type, that's terrible. > This got me thinking: how about using virtqueue_enable_cb_delayed > for this mitigation? Should work, another possible solution is interrupt coalescing, which can speed up also the case without event index. > It's pretty easy to implement - I'll send a proof of concept patch > separately. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
