Re: Fastest way to clone a hash_map

devdude <>
Tue, 29 Apr 2008 10:51:05 -0700 (PDT)
On Apr 29, 12:25 pm, Erik Wikstr=F6m <> wrote:

On 2008-04-29 17:33, devdude wrote:


I have the need to take a snapshot of a hash_map during execution
(actually transform it to a vector). This map is a shared resource
and therefore must be locked prior to any read/write operations thus I
need to minimize the amount of time the map resource is locked.

The map is defined as type <string, boost::shared_ptr<myobject>>. My
algorithm is as such:

void SnapShotToVector( vector< pair< string,
boost::shared_ptr<myobject> >& vec )

   vec.resize( map.size() );
   copy(this->map.begin(), this->map.end(),list.begin());

For about 3M elements w/in the map, I'm noticing that the resize op
takes about 150ms and the copy takes ~850ms. Is there any way to do
better? I suppose the total time doesn' t matter as it's the time the
resource is actually locked is the primary concern.

You can start by moving the vec.resize() outside the lock (actually you
should probably only have to use a reserve() and not resize()). You can
probably also use a readers/writers lock to allow reads to the map while
copying it, which might be enough if you do not perform to many
modifications on it.

Depending on how you have implemented the hash-map you might be able to
add some kind of copy on write functionality, so that if an element is
added/ remove/changed in the map it will not affect the original map but
a copy, which shares the unmodified elements with the original map.

If the readers/writers lock is not enough you need to either make the
copying faster to do something smart with the map, either way you need
some information about the implementation of the map.

Erik Wikstr=F6m

I am actually using a reader/writer lock (are there standard
implementations of r/w locks avail by chance?) but the numbers are
from stress testing using all writes so it doesn't really matter.
hash_map is google's dense_map. moving vec.resize() outside the lock
gives me problems with the copy() -- assertion occurs in the bowels of
stl. Are there strategies to "chunk" the copy through multiple
iterations over the map?

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