Re: How to make this program more efficient?

James Kanze <>
Sun, 14 Sep 2008 09:28:29 -0700 (PDT)
On Sep 13, 3:59 pm, Jerry Coffin <> wrote:

In article <d7d947bf-a634-4b9b-9d51->, says...

SUBJECT: How to make this program more efficient?

In my program, a thread will check update from server periodically and
generate a stl::map for other part of this program to read data from.
Let's name the update method as doUpdate and stl::map read methods as
getData and copyData.
Since stl::map is not thread-safe, we should do synchronization by
ourselves. A usable solution is to create a boost::mutex::scoped_lock
object in all above methods to make the access to all methods
synchronized. But since the doUpdate method will be executed
periodically and the interval may be 1 hour or longer, while all other
parts can only read the stl::map, the above solution may do
synchronization too much. We only need synchronization on all methods
when we are doUpdating.
Then is it possible to make this program more efficient?
I have thought out some other solution like add some updating flag.
But it's not safe since stl::map may be updated when the following
part of read methods is executing. And the operation on the flag may
also be interrupted by other threads.

I'd collect the data into a separate map (or perhaps vector or
whatever) and only when you've collected all the data for a
single update, copy it all into the map. This way the map only
needs to be locked for the duration of the copy.

All accesses, including reads, still need to be synchronized.

If this is still longer than you like, you can access the map
via a pointer. When you're going to do an update, you create a
copy of the map, update the copy, and then update the pointer
to refer to the newly updated copy of the map (then dispose of
the old map). This way, you only need a "lock" for the
duration of one pointer update -- but most architectures will
support an atomic swap operation without needing any other

Provided you do it in assembler. std::swap doesn't guarantee
atomicity and synchronization. (And you likely need additional
instructions for synchronization of the swap on any architecture
other than 80x86.)

This, however, is one of the rare cases where using volatile
is necessary for thread safety -- the other threads need to
treat the pointer as volatile, so as soon as the pointer
changes, they use the new pointer value and refer only to the
new map.

Volatile, at least as implemented in the compilers I know (Sun
CC, g++ and VC++) doesn't guarantee this, although arguably,
given the expressed intent behind the keyword in the C standard,
they should. You typically need some sort of fence or memory
barrier, which compilers don't generate.

James Kanze (GABI Software)
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