*********** REPLY SEPARATOR ***********
On 4/29/2003 at 11:54 AM Jae-Young Kim wrote:
>Hi, I'm developing a kernel module that enforces filtered packets to
>get delayed for a given short time. I'm using netfilter for packet
>filtering and using mod_timer() for packet delay.
>
>The kernel module holds packet buffer (skb) in a linked list and
>waits until the timer expires. If the timer expires, the module
>releases the packets.
>
>What I'm struggling is about the accuracy of timer function. Since
>default Linux timer interrupt frequency is set to 100 HZ, I know
>the smallest timer interval is 10 msec. and the one jiffy tick is
>also 10 msec. However, it looks like that there's a small amount of
>error between real-time clock and jiffy tick.
>
>In my experiment, (I set the 50msec timer for each packet and I sent
>one packet in every second), if I set 5 jiffies (= 50 msec) for my
>packet delay, the timer correctly executes the callback function
>after 5 jiffy ticks, however, the actual real-time measurment shows the
>packet delay varies between 40msec and 50msec. Even worse, the actual
>delay time variation has a trend. Please see the following data.
>
>pkt no. jiffy actual delay
>----------------------------------------
>1 5 50.2msec
>... ... ...
>300 5 45.1msec
>... ... ...
>500 5 41.6msec
>... ... ...
>566 5 40.6msec
>567 5 40.4msec
>568 5 50.3msec
>569 5 50.3msec
>... ... ...
>
Looks normal. Did someone point this at 50? Because it looks normal centered
at 45. Maybe you should try centering the delay at 50? (For a start... 5 mS for
50 mS is too much for me) At least in large lengths of time, it would then balance
out. Graph this function:
f(x) = 1/(sqrt(2*pi)) * pow(e, 0.5 * pow(x,2))
That should illustrate normality. If you make a distribution curve of actual times,
does it look like that? If so, try first to make it center around 50, then try to decrease
the standard deviation (which is reflected by having more fluctuations be closer to
the mean).
>Here, the packet delay starts from around 50msec, but gradually decreased
>to 40msec, and then abruptly adjusted to 50msec. The same
>decrese-and-abruptly-
>adjusted trend was repeated.
>
>Is there any person have experienced the same problem?
>It looks like that the accuray below 10msec is not guaranteed, but I'd
>like to
>know why this kind of trend happens (I initially thought the error should
>be
>randomly distributed between 40msec to 60msec) and how the kernel adjust
>the timer when the error term becomes over 10msec.
>
That'd be impossible. The computer is absolutely incapable of producing a
random number on its own, much less several [million]. :-p It may be semi-
random or apparently-random though.
>
>- Jay
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at http://www.tux.org/lkml/
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/