Re: [Rosegarden-devel] std::distance()

[Chris C. <ca...@al...>]

Guillaume Laurent wrote:

> Wow, that's impressive


Test code attached.  Compile it with -DMAKE_STRINGS and it
actually constructs the strings (all those appends are slow
too, so this is rather deceptive in terms of time taken to
read the file); compile it without, and it just does some
trivial integer arithmetic, giving a much more accurate
idea of how long the file read takes.  With optimisation, I
get about 14s to read the file one byte at a time and 400ms
to read it in chunks, if I don't build the strings.

Note that the MAKE_STRINGS code does some stupid hack to
ensure the correct things get appended when reading in chunks,
even if nulls are encountered.  Surely not the best way, but
arguably we shouldn't be reading binary data into strings
anyhow.

(You'll need a /tmp/test.dat of at least 30 million bytes.)


Andrew Sutton wrote:
 > it's simple math. a 30 meg
 > file is 3*(2^20) or 3145728 bytes. reading 1 byte at a time that's 3.1
 > million function calls

Well, you called it "simple".  You're a zero out -- 30 meg
is approx 30 million bytes and thus 30 million function calls,
not 3 million.  And with that many calls, you may even be
right -- allow a few dozen clock cycles per function call
and you already have several seconds to do 30 million of them
on this (poxy) 300MHz machine.


Chris