Re: gettimeofday day returning same value; possible bug or?
- From: Finn Wilcox <finnw@xxxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Tue, 24 Jul 2012 14:50:59 +0100
On 24/07/2012 13:49, Chris wrote:
I reduced the problem down to the following code:
------------------------------
local ffi = require'ffi'
ffi.cdef[[
struct timeval {
uint32_t sec;
uint32_t usec;
};
int gettimeofday(struct timeval *restrict tp, void *restrict tzp);
]]
local tp = ffi.new("struct timeval")
function gettime()
ffi.C.gettimeofday(tp, nil)
--print("***", tp.sec, tp.usec)
return tp.sec + (tp.usec / 1000000)
end
Other than the bug that Mike just confirmed, there are a few other
things you need to look out for:
1. On some 64-bit platforms (e.g. Darwin), timeval.sec is a uint64_t.
If this is the case then tp.sec + (tp.usec / 1000000) will also truncate
to a uint64_t so you will lose the fractional part and it will measure
only whole seconds.
2. tp.sec + (tp.usec / 1000000) exceeds the precision of
double-precision floating point, so you will lose some bits anyway.
If you intend to use this timer for profiling (which involves adding
lots of small intervals together) the errors will accumulate rapidly.
You might want to define a function that returns the difference in
microseconds between two timevals instead. e.g.
function elapsed_usecs(start, finish)
local s = tonumber(finish.sec - start.sec)
local us = tonumber(finish.usec - start.usec)
return s * 1000000 + us
end
Finn
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