Comment 4 for bug 1066055

Hi Stefan

I finally could revert that commit in the latest snapshot; problem was I needed to revert one later, that modified hw/virtio-serial-bus.c accordingly; after that reversion, the regression in network performance went completely away; this confirms my previous identification of the commit that caused it.

Additionally, I tested your last suggestion, to change '<' to '<=', and that didn't help; the problem was still there.

By the way, the performance gain I observed ,of about 25~30 % when using a tun/tap, was in fact just apparent, because it was result of a greater use of cpu, so it was achieved only when the host was idle; when I stress the host, with a lot of concurrent guests generating traffic, there is no gain at all.

Just for confirmation, this is the performance I get with latest snapshot (8b4a3df8081f3e6f1061ed5cbb303ad623ade66b) running wget in the guest:

$ wget -O /dev/null http://172.18.1.1/bpd.iso
--2012-10-16 09:10:18-- http://172.18.1.1/bpd.iso
Connecting to 172.18.1.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 358979584 (342M) [application/x-iso9660-image]
Saving to: `/dev/null'
100%[======================================================>] 358.979.584 29,7M/s in 11s
2012-10-16 09:10:29 (30,3 MB/s) - `/dev/null' saved [358979584/358979584]

The same wget, using the same snapshot, but with that commit reverted is:

$ wget -O /dev/null http://172.18.1.1/bpd.iso
--2012-10-16 09:15:12-- http://172.18.1.1/bpd.iso
Connecting to 172.18.1.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 358979584 (342M) [application/x-iso9660-image]
Saving to: `/dev/null'
100%[======================================================>] 358.979.584 180M/s in 1,9s
2012-10-16 09:15:14 (180 MB/s) - `/dev/null' saved [358979584/358979584]

So, as I can see, there is no doubt: that commit is the culprit; as it was intended to be related just to the quality of the source code (at least as I can see), but implied in such a cost in performance, I think it would be better to revert it.

Thank you very much, again.
Edivaldo