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SubjectRe: bug in select() (was Re: {sys_,/dev/}epoll waiting timeout)
On Mon, 27 Jan 2003, Bill Rugolsky Jr. wrote:

> Quite independent of this discussion, my boss came across this today
> while looking at some strace output:
>
> gettimeofday({1043689947, 402580}, NULL) = 0
> select(4, [0], [], [], {1, 999658}) = 0 (Timeout)
> gettimeofday({1043689949, 401857}, NULL) = 0
> gettimeofday({1043689949, 401939}, NULL) = 0
> select(4, [0], [], [], {0, 299}) = 0 (Timeout)
> gettimeofday({1043689949, 403577}, NULL) = 0
>
> Note that 1043689949.401857 - 1043689947.402580 = 1.999277.
>
> The Single Unix Specification (v2 and v3), says of select():
>
> Implementations may also place limitations on the granularity of
> timeout intervals. If the requested timeout interval requires a finer
> granularity than the implementation supports, the actual timeout
> interval shall be rounded up to the next supported value.
>
> That seems to indicate that a fix is required.

The problem is that the schedule_timeout() is not precise. So if you pass
N to such function, your sleep interval can be ]N-1, N+1[
This w/out considering other latencies but looking only at how timers are
updated ( you can call schedule_timeout() immediately before a timer tick
or immediately after ). So if we want to be sure to sleep at least the
rounded up number of jiffies, we might end up sleeping one/two jiffies
more ( and this w/out accounting other latencies ). And this will lead to
the formula ( used by poll() ) :

Tj = (Tms * HZ + 999) / 1000 + 1

( if Tms > 0 )



- Davide

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