cygrunsrv + sshd + rsync = 20 times too slow -- throttled?

[Ken Brown]

On 8/30/2021 4:14 PM, Corinna Vinschen wrote:
> Hi Ken, Hi Takashi,
> 
> On Aug 30 19:00, Corinna Vinschen wrote:
>> Ok, let's discuss this.  I added more code to my testcase and here's
>> what I see.  I dropped all data from the output which doesn't change.
>> [...]
>> - InboundQuota and OutboundQuota are always constant values and
>>    do not depend on the side the information has been queried on.
>>    That certainly makes sense.
>>
>> - WriteQuotaAvailable does not depend on the OutboundQuota, but on
>>    the InboundQuota, and very likely on the InboundQuota of the read
>>    side.  The OutboundQuota *probably* only makes sense when using
>>    named pipes with remote clients, which we never do anyway.
>>
>> The preceeding output shows that ReadDataAvailable on the read side and
>> WriteQuotaAvailable on the write side are connected.  If we write 20
>> bytes, ReadDataAvailable is incremented by 20 and WriteQuotaAvailable is
>> decremented by 20.
>>
>> So: write.WriteQuotaAvailable == InboundQuota - read.ReadDataAvailable.
>>
>> Except when a ReadFile is pending on the read side.  It's as if the
>> running ReadFile already reserved write quota.  So the write side
>> WriteQuotaAvailable is the number of bytes we can write without blocking,
>> after all pending ReadFiles have been satisfied.
>>
>> Unfortunately that doesn't really make sense when looked at it from the
>> user space.
>>
>> What that means in the first place is that WriteQuotaAvailable on the
>> write side is unreliable.  What we really need is InboundQuota -
>> read.ReadDataAvailable.  The problem with that is that the write side
>> usually has no access to the read side of the pipe.
>>
>> Long story short, I have no idea how to fix that ATM.
> 
> Well, what about keeping a duplicate of the read side handle on the
> write side just for calling NtQueryInformationFile?
> 
> Attached is an untested patch, can you have a look if that makes sense?

I probably won't get a chance to test this until tomorrow, but at a glance it 
looks great!!

Ken