Re: [Qemu-devel] [virtio-net] why does the tx virtqueue have a timer?

[Stefan Hajnoczi]

On Tue, Mar 03, 2015 at 11:21:32AM +0200, Vasile Catalin-B50542 wrote:
> 1. Why does virtio-net have a timer to handle tx virtqueue?

It's a performance optimization.  By delaying tx virtqueue processing by
150 microseconds and turning guest->host virtqueue notifications off,
QEMU is able to batch transmit.

> 2. Why isn't the virtqueue kick enough? (I see that both are set to handle
> jobs from virtqueue.)

It is enough.  The timer is not for functionality or correctness, it is
purely a performance optimization.  The optimization is a gamble, there
are workloads where the extra 150 microseconds of latency causes worse
performance.

> 3. What is the difference between regular timers and bh (bottom halves)? Why
> would you choose one over another?

A bottom half is scheduled in the main loop "as soon as possible".  A
timer has a specific duration (150 microseconds by default in this
case).

150 is a magic number.  BH is another magic number, essentially.  This
is just a bet that performance will be better if we delay a bit to allow
batching.

Hope this helps,
Stefan

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