Re: [PATCH 5/5] Stop dropping so many RX packets in tap (v3)

[Avi Kivity <avi@qumranet.com>]

Anthony Liguori wrote:
> Normally, tap always reads packets and simply lets the client drop them if it
> cannot receive them.  For virtio-net, this results in massive packet loss and
> about an 80% performance loss in TCP throughput.
>
> This patch modifies qemu_send_packet() to only deliver a packet to a VLAN
> client if it doesn't have a fd_can_read method or the fd_can_read method
> indicates that it can receive packets.  We also return a status of whether
> any clients were able to receive the packet.
>
> If no clients were able to receive a packet, we buffer the packet until a
> client indicates that it can receive packets again.
>
> This patch also modifies the tap code to only read from the tap fd if at least
> one client on the VLAN is able to receive a packet.
>
> Finally, this patch changes the tap code to drain all possible packets from
> the tap device when the tap fd is readable.
>
>  
>  #if defined(CONFIG_SLIRP)
> @@ -3970,6 +3976,8 @@ typedef struct TAPState {
>      VLANClientState *vc;
>      int fd;
>      char down_script[1024];
> +    char buf[4096];
> +    int size;
>  } TAPState;
>   

This breaks large MTUs.

How about the other way round: when the vlan consumer detects it can no 
longer receive packets, it tells that to the vlan.  When all vlan 
consumers can no longer receive, tell the producer to stop producing.  
For the tap producer, this is simply removing its fd from the read poll 
list.  When a vlan consumer becomes ready to receive again, it tells the 
vlan, which tells the producers, which then install their fds back again.

This is  a bit difficult since virtio and tap are both consumers and 
producers, but could be made to work, I think.


-- 
error compiling committee.c: too many arguments to function


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