Re: [RFC] Monotonic clock usage in buffer timestamps

["Rémi Denis-Courmont" <remi@remlab.net>]

On Wed, 2 Nov 2011 11:58:04 +0100, Daniel Glöckner <daniel-gl@gmx.net>

wrote:

>> Converting between the two can be done when making the timestamp but

it's

>> non-trivial at other times and likely isn't supported. I could be

wrong,

>> though. This might lead to e.g. timestamps that are taken before

>> switching

>> to summer time and for which the conversion is done after the switch.

>> This might be a theoretical possibility, but there might be also

>> unfavourable interaction with the NTP.

> 

> Summertime/wintertime is purely a userspace thing. UTC as returned by

> gettimeofday is unaffected by that.



Right, DST is a non-issue.



> NTP AFAIK adjusts the speed of the monotonic clock, so there is a

constant

> delta between wall clock time and clock monotonic



For NTP it depends. Simple NTP, as in ntpdate, warps the wall clock.

Full-blown NTP only adjusts the speed.



> unless there is a leap

> second or someone calls settimeofday. Applications currently using the

> wall clock timestamps should have trouble dealing with that as well.



I can think of at least three other sources of wall clock time, that could

trigger a warp:

 - GPS receiver (TAI),

 - cellular modem (NITZ),

 - and, of course, manual setting.



So if at all possible I'd much prefer monotonic over real timestamps.



-- 

Rémi Denis-Courmont

http://www.remlab.net/