Thread: General Performance questions
I have postgres set to a max connection limit of 512. I have nearly 30,000 users that will be taking a training course over the next 6 weeks. I have 3 web servers load balanced serving php content for this website. Last week I had postgres set to a connection limit of 128 and that was reached. Since then I have set it to 512 and not had a problem. However, I am unaware of any good monitoring tools that let me see live connections to the database. When I run netstat-c on the db server, it lets me see active connection in real time. Heres an example:
Tcp 0 0 server:postgres 192.168.50.x:port number ESTABLISHED
I will have anywhere from 5 to 7 of these going at any given time. However most say TIME_WAIT instead of established.
I am assuming these are connections to the database that php is using to post and retrieve data. I recently read where postgres uses one semaphore per connection in sets of 16. With that being said is one of these connections that I see in netstat-c actually representing 16 connections? If that is the case, when 6 or 7 are being displayed at once, then that would mean roughly 112 or so connections, which is great because it doesn't even come close to my limit of 512. Can anyone confirm that what I am thinking is right?
Also, is there a way to make the TIME_WAIT status shorter. It seems to exist for about 25 seconds, then goes away. However, those are connections that could easily be used.
Thank you,
Darryl
"Delao, Darryl W" <ddelao@ou.edu> writes:
> I will have anywhere from 5 to 7 of these going at any given time. However
> most say TIME_WAIT instead of established.
TIME_WAIT is a closed connection; the kernel is only remembering it for
a few seconds in case the other end requests a retransmission of the
last few outgoing bytes. This is not blocking you from creating new
sessions.
Better ways to keep track of active database sessions are grepping the
output of "ps" for postgres processes, or watching the pg_stat_activity
system view.
> Also, is there a way to make the TIME_WAIT status shorter.
Not without violating the TCP specs.
regards, tom lane
> I will have anywhere from 5 to 7 of these going at any given time. However > most say TIME_WAIT instead of established. > I am assuming these are connections to the database that php is using to > post and retrieve data. No, those connections have been closed. TIME_WAIT is part of the TCP spec. After a connection is closed the port will stay in a "TIME_WAIT" state for a short time. This is a defensive mechanism that prevents that port from being opened before enough time has elapsed to allow any stray delayed packets from the old connection to be dealt with. Without this delay another connection could be opened instantly and could incorrectly get a packet left over from the old connection. The only "live" connections are those listed as ESTABLISHED. You didn't mention what version of pg you are using but if it is a recent version try: select * from pg_stat_database; and look at the number of backends which you can get on a database by database level. You could get total connections with: select sum(numbackends) from pg_stat_database; While you are there check out all the other stuff you can see in the pg_stat* tables. Cheers, Steve
Im looking at netstat on my db server and I see a few postgres connections. I have heard that each of these connections actually represents 16 connections to postgres. Can anyone confirm this?
"Delao, Darryl W" <ddelao@ou.edu> writes:
> Im looking at netstat on my db server and I see a few postgres connections.
> I have heard that each of these connections actually represents 16
> connections to postgres. Can anyone confirm this?
Where did you hear that? It's nonsense.
regards, tom lane