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Re: Possible Bug: high CPU usage for stats collector in 8.2

From: Andrew Dunstan <andrew(at)dunslane(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Darcy Buskermolen <darcyb(at)commandprompt(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Possible Bug: high CPU usage for stats collector in 8.2
Date: 2007-03-01 20:20:05
Message-ID: 45E73575.90706@dunslane.net (view raw or flat)
Thread:
Lists: pgsql-hackers
Tom Lane wrote:
> Darcy Buskermolen <darcyb(at)commandprompt(dot)com> writes:
>   
>> I'm observing high CPU usage (95%) of a 2.6GHz opteron by the stats
>> collector on an 8.2.3 box  investigation has lead me to belive that the
>> stats file is written a lot more often that once every 500ms  the
>> following shows this behavior.
>>     
>
> Looks like someone broke the usage of PGSTAT_STAT_INTERVAL between 8.1
> and 8.2 ... it's waiting 500 microseconds, not the intended 500
> milliseconds.
>   

Good catch. I am also a bit dubious about this code:

        input_fd.fd = pgStatSock;
        input_fd.events = POLLIN | POLLERR;
        input_fd.revents = 0;

        if (poll(&input_fd, 1, PGSTAT_SELECT_TIMEOUT * 1000) < 0)
        {
            if (errno == EINTR)
                continue;
            ereport(ERROR,
                    (errcode_for_socket_access(),
                     errmsg("poll() failed in statistics collector: %m")));
        }

        got_data = (input_fd.revents != 0);


AIUI you are not supposed to put POLLERR in the events field. We should 
probably be setting POLLIN | POLLPRI,  and we should also probably check 
exactly what event was returned in revents.

cheers

andrew




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