On Thu, Aug 11, 2011 at 15:06, Nikos Chantziaras <[email protected]> wrote: > On 08/11/2011 10:35 AM, Pandu Poluan wrote: >> >> On Thu, Aug 11, 2011 at 14:27, Nikos Chantziaras<[email protected]> wrote: >>> >>> On 08/11/2011 09:56 AM, Pandu Poluan wrote: >>>> >>>> Just wondering, is it possible to specify a custom timer frequency? >>>> E.g., HZ=500 instead of one of the canned values (100, 250, 300, >>>> 1000). >>> >>> It is possible, but it's a bad idea because non-standard values can >>> result >>> in driver breakage. Some code assumes specific timer granularities >>> (100Hz = >>> 10ms, 250Hz = 4ms, etc). This usually happens with values above 1000Hz, >>> so >>> it might be possible to experiment with non-standard sub-1000Hz values. >>> >>> But why do you want a custom value anyway? >>> >> >> Well, for a firewall, I've calculated (gathered and extrapolated from >> a lot of sources), the latency per-packet is usually less than 1 ms, >> at worst still less than 2 ms. >> >> Thus, setting the timer freq to 100 Hz (as suggested for 'normal' >> server load) means the timeslice is way too long (10 ms per time >> slice). >> >> So, I speculate that better -- and uniform -- performance will be >> achieved with a timer freq of 500 Hz. >> >> Of course, this is a wild speculation/guess from me. I never quite >> understand netfilter/xtables' relation to the timeslices, so I may be >> talking nonsense :-) > > This assumes that networking is dependent on the timer interrupt, which > doesn't seem to be the case; going from 100Hz to 1000Hz will not result in > network latencies dropping by 9ms. I know because I tried it on a server. >
That's what I was concerned of. > The only case where network latency improved with a higher HZ was with a > game server (Counter-Strike) and that's only because that game's server > component is timer dependent when calculating the game world (to do 1000 > updates per second it needs a HZ value of 1000). So that application has > some real-time demands, meaning a high HZ value helps. Otherwise, you can > stick to 100Hz on a server. Higher values won't change anything and will > only reduce throughput (though not by much anyway, which is why some people > set 1000Hz even on servers.) > Hmmm... I *do* feel better response (interaction-wise via SSH) if I use >100, so I think I'll settle for 250 Hz. Thanks for the explanation! Rgds, -- FdS Pandu E Poluan ~ IT Optimizer ~ • Blog : http://pepoluan.tumblr.com • Linked-In : http://id.linkedin.com/in/pepoluan
