On Fri, Jul 19, 2019 at 04:51:00PM +0800, Jason Wang wrote: > > On 2019/7/19 下午4:39, Stefano Garzarella wrote: > > On Fri, Jul 19, 2019 at 04:21:52PM +0800, Jason Wang wrote: > > > On 2019/7/19 下午4:08, Stefano Garzarella wrote: > > > > On Thu, Jul 18, 2019 at 07:35:46AM -0400, Michael S. Tsirkin wrote: > > > > > On Thu, Jul 18, 2019 at 11:37:30AM +0200, Stefano Garzarella wrote: > > > > > > On Thu, Jul 18, 2019 at 10:13 AM Michael S. Tsirkin<mst@xxxxxxxxxx> wrote: > > > > > > > On Thu, Jul 18, 2019 at 09:50:14AM +0200, Stefano Garzarella wrote: > > > > > > > > On Wed, Jul 17, 2019 at 4:55 PM Michael S. Tsirkin<mst@xxxxxxxxxx> wrote: > > > > > > > > > On Wed, Jul 17, 2019 at 01:30:29PM +0200, Stefano Garzarella wrote: > > > > > > > > > > If the packets to sent to the guest are bigger than the buffer > > > > > > > > > > available, we can split them, using multiple buffers and fixing > > > > > > > > > > the length in the packet header. > > > > > > > > > > This is safe since virtio-vsock supports only stream sockets. > > > > > > > > > > > > > > > > > > > > Signed-off-by: Stefano Garzarella<sgarzare@xxxxxxxxxx> > > > > > > > > > So how does it work right now? If an app > > > > > > > > > does sendmsg with a 64K buffer and the other > > > > > > > > > side publishes 4K buffers - does it just stall? > > > > > > > > Before this series, the 64K (or bigger) user messages was split in 4K packets > > > > > > > > (fixed in the code) and queued in an internal list for the TX worker. > > > > > > > > > > > > > > > > After this series, we will queue up to 64K packets and then it will be split in > > > > > > > > the TX worker, depending on the size of the buffers available in the > > > > > > > > vring. (The idea was to allow EWMA or a configuration of the buffers size, but > > > > > > > > for now we postponed it) > > > > > > > Got it. Using workers for xmit is IMHO a bad idea btw. > > > > > > > Why is it done like this? > > > > > > Honestly, I don't know the exact reasons for this design, but I suppose > > > > > > that the idea was to have only one worker that uses the vring, and > > > > > > multiple user threads that enqueue packets in the list. > > > > > > This can simplify the code and we can put the user threads to sleep if > > > > > > we don't have "credit" available (this means that the receiver doesn't > > > > > > have space to receive the packet). > > > > > I think you mean the reverse: even without credits you can copy from > > > > > user and queue up data, then process it without waking up the user > > > > > thread. > > > > I checked the code better, but it doesn't seem to do that. > > > > The .sendmsg callback of af_vsock, check if the transport has space > > > > (virtio-vsock transport returns the credit available). If there is no > > > > space, it put the thread to sleep on the 'sk_sleep(sk)' wait_queue. > > > > > > > > When the transport receives an update of credit available on the other > > > > peer, it calls 'sk->sk_write_space(sk)' that wakes up the thread > > > > sleeping, that will queue the new packet. > > > > > > > > So, in the current implementation, the TX worker doesn't check the > > > > credit available, it only sends the packets. > > > > > > > > > Does it help though? It certainly adds up work outside of > > > > > user thread context which means it's not accounted for > > > > > correctly. > > > > I can try to xmit the packet directly in the user thread context, to see > > > > the improvements. > > > > > > It will then looks more like what virtio-net (and other networking device) > > > did. > > I'll try ASAP, the changes should not be too complicated... I hope :) > > > > > > > > > > Maybe we want more VQs. Would help improve parallelism. The question > > > > > would then become how to map sockets to VQs. With a simple hash > > > > > it's easy to create collisions ... > > > > Yes, more VQs can help but the map question is not simple to answer. > > > > Maybe we can do an hash on the (cid, port) or do some kind of estimation > > > > of queue utilization and try to balance. > > > > Should the mapping be unique? > > > > > > It sounds to me you want some kind of fair queuing? We've already had > > > several qdiscs that do this. > > Thanks for pointing it out! > > > > > So if we use the kernel networking xmit path, all those issues could be > > > addressed. > > One more point to AF_VSOCK + net-stack, but we have to evaluate possible > > drawbacks in using the net-stack. (e.g. more latency due to the complexity > > of the net-stack?) > > > Yes, we need benchmark the performance. But as we've noticed, current vsock > implementation is not efficient, and for stream socket, the overhead should > be minimal. The most important thing is to avoid reinventing things that has > already existed. Got it. I completely agree with you, and I want to avoid reinventing things (surely in a worse way). But the idea (suggested also by Micheal) to discover how fast can go a new protocol separate from the networking stack, is quite attractive :) Thanks, Stefano
