Re: How to make this program more efficient?
On Sep 13, 2:33 pm, courp...@gmail.com wrote:
On 13 sep, 12:28, Jon Harrop <j...@ffconsultancy.com> wrote:
James Kanze wrote:
On Sep 13, 3:18 am, Bill David <billdavi...@gmail.com> 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. 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.
This is semantically wrong. In fact, many programmers believe
that "synchonizing threads to a shared ressource" necessarily
means that a lock of some sort is needed, whereas there are
many synchronization techniques that avoids the use of locks.
What you are refering to seems to be an instance of the RCU
algorithm, a kind of lock-free *synchronization* algorithm.
It is certainly possible to design some types of containers to
be lock-free. If you're using the standard containers, however,
and just wrapping them, I don't think that a lock-free algorithm
can be made to work.
And of course, as you say, it's a lot more work, and rarely
necessary.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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