On Thu, 23 Dec 2004, Jeff Behl wrote:
As a follow up to the below (original message at the very bottom), I
installed a load balancer in front of the machines which terminates the
tcp connections from clients and opens up a few, persistent connections
to each server over which requests are pipelined. In this scenario
everything is copasetic:
I'm not very familiar with Squid's architecture, but I would anticipate
that what you're seeing is that the cost of additional connections served
in parallel is pretty high due to the use of processes. Specifically: if
each TCP connection being served gets its own process, and there are a lot
of TCP connections, you'll be doing a lot of process forking, context
switching, exceeding cache sizes, etc. With just a couple of connections,
even if they're doing the same "work", the overhead is much lower. Depending on how much time you're willing to invest in this, we can
probably do quite a bit to diagnose where the cost is coming from and look
for any specific problems or areas we could optimize.
It must not be this. Squid is mostly a single process system, with scheduling based on descriptors and select/poll. Recent versions added some parallelism in other processes, but just for file reading/writing (diskd) and regular expression processing for ACLs. Even DNS, which previously ran on blocking I/O in secondary processes now run internally in the select/poll scheduler.
I also have some experience in older versions of squid, in which the same machine running the same version of squid, and changing Linux for FreeBSD raised the maximum simultaneus conection limit.
I might start by turning on kernel profiling and doing a profile dump under load. Be aware that turning on profiling uses up a lot of CPU itself, so will reduce the capacity of the system. There's probably documentation elsewhere, but the process I use to set up profiling is here:
I did not make any tests on this, but I would expect profiling to fail, since every step of the scheduler is very small, and deals with the smallest I/O available at that time.
Indeed, based on the original report I would search for some optimization on descriptor searching in poll or select, whichever squid has chosen to use on FreeBSD (probably select, looking at the top output). This is one of the crucial points on squid performance. The other one is disk access, for sure, but the experimente describe would not change disk access patterns, would it?
http://www.watson.org/~robert/freebsd/netperf/profile/
Note that it warns the some results may be incorrect on SMP. I think it would be useful to give it a try anyway just to see if we get something useful.
As I said before, beeing a single process scheduler, squid does not gain much from SMP. The secondary processes would benefit from the extra CPU, though. Maybe interrupt processing also, if the giant lock does not interfere in any part of the processing path.
As a final question: other than CPU consumption, do you have a reliable way to measure how efficiently the system is operating -- in particular, how fast it is able to serve data? Having some sort of metric for performance can be quite useful in optimizing, as it can tell us whether
One thing I fail to measure in FreeBSD is the reason for delays in disk access times. How can I prove that the delay is on disk, and determine how to optimize it? systat -v is very useful, but does not give me all answers.
last pid: 3377; load averages: 0.12, 0.09, 0.08 up 0+17:24:53 10:02:13 31 processes: 1 running, 30 sleeping CPU states: 5.1% user, 0.0% nice, 1.8% system, 1.2% interrupt, 92.0% idle Mem: 75M Active, 187M Inact, 168M Wired, 40K Cache, 214M Buf, 1482M Free Swap: 4069M Total, 4069M Free
PID USERNAME PRI NICE SIZE RES STATE C TIME WCPU CPU COMMAND 474 squid 96 0 68276K 62480K select 0 53:38 16.80% 16.80% squid 311 bind 20 0 10628K 6016K kserel 0 12:28 0.00% 0.00% named
Jonny
-- João Carlos Mendes Luís - Networking Engineer - [EMAIL PROTECTED] _______________________________________________ freebsd-net@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-net To unsubscribe, send any mail to "[EMAIL PROTECTED]"
