[gdal-dev] Multithreading with OGR

[Martin Dobias]

Hi Ed

On Wed, Jun 23, 2010 at 3:18 PM, Grissom, Edward (Ed)
<ed.grissom at intergraph.com> wrote:
> In a previous message, Martin said:
>
>> So my only problem is to avoid concurrent reads from
>> both main thread and worker thread. How can be this
>> best accomplished without serializing the access?
>
> Your reads are eventually going to be serialized by the disk controller
> anyway, so what is the problem with serializing the access ?   Just do
> it on a small enough scale that no thread gets starved.

You're right, but that is not what I had in my mind, sorry for being
unclear. When rendering from worker thread, I have to avoid this kind
of situations (considering one layer):

1. worker thread calls layer's ResetReading, then starts issuing
GetNextFeature calls
2. before worker thread is done, main thread calls layer's
ResetReading and also starts reading features
3. both main thread and worker thread do GetNextFeature calls, but
both get invalid results

This can happen e.g. when user tries to identify some features on map
while rendering is in progress. With the "serialization of access" I
meant serializing the calls: if main thread was going to read
features, it would first wait until rendering of that layer finishes
in worker thread. Clearly, with serialized access, there wouldn't be
much added value when using threading.

So, my main question is, if opening the same layer for worker thread
is a safe operation. That is, if both main and worker thread will
access (with GetNextFeature calls) their own copy of the layer,
whether they both end up with correct data.

Regards
Martin