Re: Slow ICP under high loads (squid-1.2beta20 & 21)

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Unless someone can come up with a good algorithm, maybe we should make
this a run-time tweakable. Just another number for people to ignore the
documentation on and set to something ludicrous, yeah?

D

Stewart Forster wrote:
>
> For some time now I've been encountering very slow ICP responses from caches
> under high load (sustained > 30 TCP & > 100 ICP requests/sec). I've managed
> to trace this down to some code that was added a little while ago in the
> effort to reduce CPU usage.
>
> in comm.c:comm_poll (and comm_select if you use select)
>
> if (revents & (POLLRDNORM | POLLIN | POLLHUP | POLLERR)) {
> debug(5, 6) ("comm_poll: FD %d ready for reading\n", fd);
> if ((hdl = fd_table[fd].read_handler)) {
> fd_table[fd].read_handler = NULL;
> hdl(fd, fd_table[fd].read_data);
> }
> ** if ((incoming_counter++ & 15) == 0)
> ** comm_poll_incoming();
> }
> if (revents & (POLLWRNORM | POLLOUT | POLLHUP | POLLERR)) {
> debug(5, 5) ("comm_poll: FD %d ready for writing\n", fd);
> if ((hdl = fd_table[fd].write_handler)) {
> fd_table[fd].write_handler = NULL;
> hdl(fd, fd_table[fd].write_data);
> }
> ** if ((incoming_counter++ & 15) == 0)
> ** comm_poll_incoming();
>
> ie. the above code calls comm_poll_incoming every 16 read or write
> requests. I've tried lower values, but the only one that seems to work
> okay is if I remove the "if ((incoming_counter++ & 15) == 0)" test and just
> call comm_poll_incoming for EVERY read or write event. This then results in
> timely ICP transactions and packets not being dropped. I've tested this in
> a production environment to a sustained 65 TCP hits/sec and 200 ICP hits/sec
> with no ICP slowdown evident yet.
>
> I'm not sure what the best fix is, but the "every 16" approach is only
> fine for low-medium loaded caches.
>
> Stew.
>
> --
> Stewart Forster (Snr. Development Engineer)
>
> connect.com.au pty ltd, Level 9, 114 Albert Rd, Sth Melbourne, VIC 3205, Aust.
> Email: slf@connect.com.au Phone: +61 3 9251-3684 Fax: +61 3 9251-3666

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