colo: qemu 4.2.0 vs. qemu 5.0.0-rc2 performance regression

[Lukas Straub]

Hello Everyone,
I did some Benchmarking with iperf3 and memtester (to dirty some guest memory)
of colo performance in qemu 4.2.0 and in qemu 5.0.0-rc2
with my bugfixes on top.( 
https://lists.nongnu.org/archive/html/qemu-devel/2020-04/msg01432.html )

I have taken the average over 4 runs.
Client-to-server tcp bandwidth rose slightly from ~83.98 Mbit/s to ~89.40 Mbits.
Server-to-client tcp bandwidth fell from ~9.73 Mbit/s to ~1.79 Mbit/s.
Client-to-server udp bandwidth stayed the same at 1.05 Mbit/s
and jitter rose from ~5.12 ms to ~10.77 ms.
Server-to-client udp bandwidth fell from ~380.5 Kbit/s to ~33.6 Kbit/s
and jitter rose from ~41.74 ms to ~83976.15 ms (!).

I haven't looked closely into it, but i think
0393031a16735835a441b6d6e0495a1bd14adb90 "COLO: Optimize memory back-up process"
is the culprint as it reduces vm downtime for the checkpoints but increases
the overall checkpoint time and we can only release miscompared primary packets
after the checkpoint is completely finished.

Another thing that I noticed: With 4.2.0, the secondary qemu uses thrice
the amount of gest memory. With 5.0.0-rc2 it's just double the amount of
guest memory. So maybe the ram cache isn't working properly?

Regards,
Lukas Straub

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