On Mon, Jul 29, 2019 at 05:36:56PM +0200, Stefano Garzarella wrote: > On Mon, Jul 29, 2019 at 10:04:29AM -0400, Michael S. Tsirkin wrote: > > On Wed, Jul 17, 2019 at 01:30:26PM +0200, Stefano Garzarella wrote: > > > Since virtio-vsock was introduced, the buffers filled by the host > > > and pushed to the guest using the vring, are directly queued in > > > a per-socket list. These buffers are preallocated by the guest > > > with a fixed size (4 KB). > > > > > > The maximum amount of memory used by each socket should be > > > controlled by the credit mechanism. > > > The default credit available per-socket is 256 KB, but if we use > > > only 1 byte per packet, the guest can queue up to 262144 of 4 KB > > > buffers, using up to 1 GB of memory per-socket. In addition, the > > > guest will continue to fill the vring with new 4 KB free buffers > > > to avoid starvation of other sockets. > > > > > > This patch mitigates this issue copying the payload of small > > > packets (< 128 bytes) into the buffer of last packet queued, in > > > order to avoid wasting memory. > > > > > > Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> > > > Signed-off-by: Stefano Garzarella <sgarz...@redhat.com> > > > > This is good enough for net-next, but for net I think we > > should figure out how to address the issue completely. > > Can we make the accounting precise? What happens to > > performance if we do? > > > > In order to do more precise accounting maybe we can use the buffer size, > instead of payload size when we update the credit available. > In this way, the credit available for each socket will reflect the memory > actually used. > > I should check better, because I'm not sure what happen if the peer sees > 1KB of space available, then it sends 1KB of payload (using a 4KB > buffer). > The other option is to copy each packet in a new buffer like I did in > the v2 [2], but this forces us to make a copy for each packet that does > not fill the entire buffer, perhaps too expensive. > > [2] https://patchwork.kernel.org/patch/10938741/ >
So one thing we can easily do is to under-report the available credit. E.g. if we copy up to 256bytes, then report just 256bytes for every buffer in the queue. > > Thanks, > Stefano
