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- Subject: Re: Lua for Time Travel (!)
- From: Sean Conner <sean@...>
- Date: Wed, 25 Jan 2017 15:31:12 -0500
It was thus said that the Great William Ahern once stated:
> On Wed, Jan 25, 2017 at 01:21:36PM +0000, Joseph Manning wrote:
> > Dear All,
> > Have you ever wondered how the Lua Team manage to retain their
> > ever-youthful good looks?
> > Do you want to become mega-rich like Biff in Back To The Future II,
> > by placing bets when you already know the outcome?
> > Or would you simply like to run your programs so fast that they
> > actually finish before they've even started?
> > Well, these are all now possible, by using Lua for Time Travel !!!
> > ( ... takes his medication, and a semblance of sanity returns ... )
> > So, I was timing a section of code by wrapping it in calls to
> > 'os.clock( )', and under certain conditions, the second such call
> > returned a value that was substantially smaller than the first --
> > in fact, it returned a negative value, which should never happen.
> Here's a defect ticket for POSIX discussing the overflow issue.
> The issue was brought up with the C committee. (See DR 437 in
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1764.pdf) But they want to
> wait and see what solutions the POSIX people suggest. Alas, issue #686
> hasn't seen any activity since then.
> In C I just explicitly type cast clock_t values (each operand individually)
> to unsigned long, and everything just magically works because of modulo
> arithmetic. But I'm making assumptions about the platform--that it's two's
> complement and that unsigned long is the corresponding unsigned type for
> clock_t. I really only use it for debugging and quick benchmarking. But as
> mentioned in that ticket there's no [easy] portable way to figure out which
> unsigned type to convert to. For Lua you'd probably have to use a function
> with conditional logic, just like a portable C solution would likely use.
> Usually for this sort of thing it's best to use the system's monotonic
> clock--clock_gettime(CLOCK_MONOTONIC) on POSIX systems. On modern Linux
> systems the granularity and overhead will be the same as clock().
> clock_gettime returns a struct timespec, which is also now the preferred
> type for time intervals in the latest C standard, C11.
>  clock() is implemented as clock_gettime(CLOCK_PROCESS_CPUTIME_ID). And
> both CLOCK_PROCESS_CPUTIME_ID and CLOCK_MONOTONIC perform pretty much the
> same operations--query the CPU TSC, read some cached values, perform the
I have a Lua module  that calls clock_gettime(), but there's no
LuaRocks spec for it.