[Qgis-developer] [Server] performance questions

[Andrea Peri]

I guess a more time consuming parameter is the number of vertex in the
feature involved in the map rather than the layer numbers.

A.
Il 22 apr 2016 3:27 PM, "Jörg Habenicht" <jh at mwerk.net> ha scritto:

> Hi Andreas, dear list,
>
> sorry for the warm up of this old topic.
>
> There is in deed a need to enable multi threading in Qgis-Server.
>
> In our company we use around 60 different layer to lay some topics on
> the map. This may be an extreme example to read, but quite common in our
> environment, i.e. we can not lower this number significantly.
>
> A warmed up QGis-Server process which already read and cached the
> Configuration file qgis-xy.conf still needs around 20 seconds to
> calculate the complete resulting map picture.
>
> During this calculation there is only one thread working, serializing
> the DB-requests, calculation the layer picture and mapping the picture
> on the resulting picture.
> The process is running on a multi core server. If we can utilize the
> idle cores to do useful work, this can be an easy example to speed up
> the process (significant).
>
> I'd like to share my developing experience to do this change, but I
> would need further information and some support to do some useful patches.
>
> I have scanned the git-Qgis sources (commit
> 81744ecf90a8f6c1e2e94fdacb07e3dca6987dcc) for a suitable multithreading
> entry. Found QgsMapRenderer::render() because of the line "while (
> li.hasPrevious() )". It seems in this loop the layers are painted in
> serial onto the resulting picture.
> If I can build an auto parallel computing with QtConcurrent in this
> place there seems to be the most gain. I mean to parallelize the loop
> with concurrent DB requests, concurrent painting in separate pictures
> and in serial map the single pictures onto the result picture.
>
> Right now I don't know if I can do DB requests in parallel, or where the
> code for database requests is buried.
> And how about the calls inside the loop, are there side effects
> preventing multi threading?
>
>
> Do you got hints, flames or suggestions?
>
>
> cu
> Jörg
>
>
> Am 23.02.2016 um 16:39 schrieb Neumann, Andreas:
> > Hi Régis,
> >
> > QGIS server is single-threaded. You can't simply turn on multi-threaded
> > rendering in QGIS server. The other issue is that each Apache FCGI
> > process has it's own set of cache. So if you have 5-10 parallel Apache
> > threads or processes each thread/process has it's own cache. If your
> > client hits a thread that has the cache unititialized you will have wait
> > longer until this cache is fillled.
> >
> > I once asked Marco whether one could
> >
> > a) have a shared cache for all Apache threads or processes
>
> There is no shared cache, because Apache spawns QGis-Processes with its
> own memory layout. these Processes know nothing about its neighbor, and
> you have a hard time to exchange data between all processes.
> How about a scheduler, which assigns requests to warmed up Qgis-processes?
>
> >
> > b) support multi-threaded rendering
> >
> > But he said it would be very complicated to implement a) and b), if not
> > impossible. Maybe other devs have other views. Note that was about 2
> > years ago, when I last discussed this with Marco.
> >
> > ---------------------
> >
> > Nevertheless I believe there would be room for performance improvements
> > in QGIS server - if interested parties share their efforts (be it dev
> > time or financial resources).
> >
> [snip]
> >
> > Andreas
> >
> [snip]
> >
> >
> >
> >
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>
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