Re: MSDN volatile sample
George wrote:
:: Thanks Bo,
::
::
:: I agree and understand all of your comments, except that the below
:: comments. I am not sure what will not work in VC6 and VC7?
Using volatile variables for synchronization are not guaranteed. That
is an extension introduced in the lastest VC++ releases.
:: Actually, I am using the new Visual Studio 2008. :-)
Then it will work, as long as you don't move your code to another
compiler!
I have a creeping feeling that you are too busy examining each
individual tree, that you might be missing the idea of a forest.
Most often there is no need for determining the fastest way to share
data between threads, but rather how you design the program so that
there is no need to share it.
MSDN's volatile trick perhaps works reasonable well on a 2-4 core x86,
but might be extremely expensive on, for example, a system with a few
hundred Itanium processors. If they all have to synchronize their
on-chip caches each time you write to a volatile variable, that would
keep them pretty busy and stopping them from doing anything useful.
Bo Persson
::
::: MSDN describes Microsoft's implementation of C++, it is not a
::: general description. Specifically, you are reading the spec for
::: VC++ 8.0, which has an EXTENSION for volatile variables, making
::: the guarantees used in the example.
:::
::: If you go back to older versions, like VC6 or VC7, it will not
::: work (or at least there are no guarantees). If you move the code
::: to another compiler, you will have to closely read its
::: documentation, to see if the guarantees still hold.
::
::
:: regards,
:: George
::
:: "Bo Persson" wrote:
::
::: George wrote:
::::: Thanks Alex,
:::::
:::::
::::: I am wondering if you use mutex or other technologies, like
::::: critical section or something, to synchronize/share the
::::: cross-thread variable, do you think we still need the volatile
::::: keyword?
:::
::: No, not in this case.
:::
::: The general use for volatile is in interfacing with external
::: hardware. A pointer to volatile char could be a port on video
::: board, or something. The volatile keyword tells the compiler that
::: each read from that address could give a different value, so it
::: has to actually be read each time, and not cached and reused.
:::
:::::
::::: I have this concern is because some other people in this
::::: discussion mentioned that volatile is an alternative way to
::::: share data between threads (alternatives are like mutex), so if
::::: other approaches like critical section or mutex is used,
::::: volatile is no need.
:::
::: Right.
:::
:::::
::::: But I can not find any formal statements about this point from
::::: MSDN.
:::
::: MSDN describes Microsoft's implementation of C++, it is not a
::: general description. Specifically, you are reading the spec for
::: VC++ 8.0, which has an EXTENSION for volatile variables, making
::: the guarantees used in the example.
:::
::: If you go back to older versions, like VC6 or VC7, it will not
::: work (or at least there are no guarantees). If you move the code
::: to another compiler, you will have to closely read its
::: documentation, to see if the guarantees still hold.
:::
::: That's one good reason for not using volatile this way!
:::
:::
:::
::: Bo Persson