Re: I/O performance of VirtIO

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> On 10/23/2009 12:06 AM, Alexander Graf wrote:
>>
>> Am 22.10.2009 um 18:29 schrieb Avi Kivity <avi@redhat.com>:
>>
>>> On 10/13/2009 08:35 AM, Jan Kiszka wrote:
>>>> It can be particularly slow if you use in-kernel irqchips and the
>>>> default NIC emulation (up to 10 times slower), some effect I always
>>>> wanted to understand on a rainy day. So, when you actually want -net
>>>> user, try -no-kvm-irqchip.
>>>>
>>>
>>> This might be due to a missing SIGIO or SIGALRM; -no-kvm-irqchip
>>> generates a lot of extra signals and thus polling opportunities.
>>
>> Isn't that what dedicated io threads are supposed to solve?
>>
> 
> No.  Dedicated I/O threads provide parallelism.  All latency needs is to
> have SIGIO sent on all file descriptors (or rather, in qemu-kvm with
> irqchip, to have all file descriptors in the poll() call).
> 
> Jan, does slirp add new connections to the select set?
> 

It should do so in slirp_select_fill (it iterates over all TCP&UDP
sockets of all instances). I think without doing this, slirp wouldn't
receive a single bit at all (no activity without FD_ISSET).

Jan


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