Re: How to make this program more efficient?

James Kanze <>
Sun, 14 Sep 2008 09:15:28 -0700 (PDT)
On Sep 13, 12:28 pm, Jon Harrop <> wrote:

James Kanze wrote:

On Sep 13, 3:18 am, Bill David <> wrote:

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.

If there's any thread modifying the object, then all
accesses must be synchronized. Period.

That is incorrect.

Not according to Posix. And not on some of the more recent

Caches are an obvious counter example where the writer can
write a single word (a pointer to the updated immutable cache)
atomically and readers can read the word atomically safely
without any locking at all.

First, there's formally no guarantee about writing even a single
word. But that's not the issue; the issue is one of ordering.
And the upcoming C++ standard will be clear here: if more than
one thread accesses the container, and any thread modifies it,
then all accesses must be synchronized.

James Kanze (GABI Software)
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