On 01/19/2018 05:53 PM, Konstantin Knizhnik wrote:
>
>
> On 19.01.2018 19:28, Pavel Stehule wrote:
>>
>>
>> When I've been thinking about adding a built-in connection
>> pool, my
>> rough plan was mostly "bgworker doing something like
>> pgbouncer" (that
>> is, listening on a separate port and proxying everything to
>> regular
>> backends). Obviously, that has pros and cons, and probably
>> would not
>> work serve the threading use case well.
>>
>>
>> And we will get the same problem as with pgbouncer: one process
>> will not be able to handle all connections...
>> Certainly it is possible to start several such scheduling
>> bgworkers... But in any case it is more efficient to multiplex
>> session in backend themselves.
>>
>>
>> pgbouncer hold all time client connect. When we implement the
>> listeners, then all work can be done by worker processes not by listeners.
>>
>
> Sorry, I do not understand your point.
> In my case pgbench establish connection to the pgbouncer only once at
> the beginning of the test.
> And pgbouncer spends all time in context switches (CPU usage is 100% and
> it is mostly in kernel space: top of profile are kernel functions).
> The same picture will be if instead of pgbouncer you will do such
> scheduling in one bgworker.
> For the modern systems are not able to perform more than several
> hundreds of connection switches per second.
> So with single multiplexing thread or process you can not get speed more
> than 100k, while at powerful NUMA system it is possible to achieve
> millions of TPS.
> It is illustrated by the results I have sent in the previous mail: by
> spawning 10 instances of pgbouncer I was able to receive 7 times bigger
> speed.
>
AFAICS making pgbouncer multi-threaded would not be hugely complicated.
A simple solution would be a fixed number of worker threads, and client
connections randomly assigned to them.
But this generally is not a common bottleneck in practical workloads (of
course, YMMV).
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
