Re: reduce networking latency

[David Xu <davidxu06@gmail.com>]

2014-10-15 14:30 GMT-04:00 David Xu <davidxu06@gmail.com>:
> 2014-09-29 5:04 GMT-04:00 Michael S. Tsirkin <mst@redhat.com>:
>> On Wed, Sep 24, 2014 at 02:40:53PM -0400, David Xu wrote:
>>> Hi Michael,
>>>
>>> I found this interesting project from KVM TODO website:
>>>
>>> allow handling short packets from softirq or VCPU context
>>>  Plan:
>>>    We are going through the scheduler 3 times
>>>    (could be up to 5 if softirqd is involved)
>>>    Consider RX: host irq -> io thread -> VCPU thread ->
>>>    guest irq -> guest thread.
>>>    This adds a lot of latency.
>>>    We can cut it by some 1.5x if we do a bit of work
>>>    either in the VCPU or softirq context.
>>>  Testing: netperf TCP RR - should be improved drastically
>>>           netperf TCP STREAM guest to host - no regression
>>>
>>> Would you mind saying more about the work either in the vCPU or
>>> softirq context?
>>
>> For TX, we might be able to execute it directly from VCPU context.
>> For RX, from softirq context.
>>

Do you mean for RX, we directly put data to a shared buffer accessed
by guest VM bypassing the IO thread? For TX, in vCPU context network
data is added to the shared buffer and kick host IRQ to send them?

>>> Why it is only for short packets handling?
>>
>> That's just one idea to avoid doing too much work like this.
>>
>> Doing too much work in VCPU context would break pipelining,
>> likely degrading stream performance.
>> Work in softirq context is not accounted against the correct
>> cgroups, doing a lot of work there will mean guest can steal
>> CPU from other guests.
>>
>>> Thanks a
>>> lot!
>>>
>>>
>>> Regards,
>>>
>>> Cong