[Qt-interest] QAbstractListModel Scrolling Performance Issue

[Scott Aron Bloom]

Try to simplfy the problem a bit..

First... I would simply create a dummy model that had 40k rows, no SQL.
See if the problem still exists..

The main trick is going to be to represent the system your ::data() and
model queries in the same way.

If that doesn't show the problem you can look into the SQL side of
things... Maybe there is a delay your not thinking of...

If the delay is still there, then focus on the model and the view side
of things.

BTW What type of view are you using?

Have your tried using QSqlQueryModel (or one of its derivatives) as the
model?

Scott
> -----Original Message-----
> From: qt-interest-bounces at trolltech.com [mailto:qt-interest-
> bounces at trolltech.com] On Behalf Of Stephan Rose
> Sent: Wednesday, December 03, 2008 9:08 AM
> To: Qt-interest at trolltech.com
> Subject: [Qt-interest] QAbstractListModel Scrolling Performance Issue
> 
> First off, some background information.
> 
> My original model, also based on a QAbstractListModel, used a SQL
Query to
> obtain data from a MySQL Database. In the data(..) function I'd seek
to the
> appropriate record in the resultset and then return the requested
column data.
> 
> This worked great but initial scrolling performance was poor. If for
instance
> I'd have a resultset with 40,000 records it would take a couple
seconds to
> scroll all the way from the top to bottom by dragging on the
scrollbar. I
> could definitely feel a major lag. However, having fully scrolled from
top to
> bottom at least one time, from that point on scrolling is perfectly
smooth.
> 
> So, this week I implemented a new data server which runs on it's own
thread in
> the background and created a new model to work with that server. This
new
> model submits a query to the data server which is then added to a task
queue
> and eventually processed.
> 
> Along with submitting the query, the model also submits a slot that is
> connected to a signal of the data server's task. When the query is
then
> finally executed, it batches a bunch of QSqlRecords (currently 1,000
at a
> time) and then emits a data received event with a QList containing
those
> records. This also helped speed up query execution as I can now set
> forwardOnly to true.
> 
> The slot implemented in the model then takes these records, and
together with
> beginInsertRows and endInsertRows appends these to it's own internal
QList of
> records.
> 
> Works wonderfully, especially with larger queries that used to
momentarily
> freeze the UI. However, the scrolling issue previously described still
exists.
> 
> Using the same query that returned the 40,000 records as described in
the
> initial example, I still get the problem that if I want to scroll all
the way
> to the bottom there is a visible lag. I've verified that the data
server has
> finished executing the query and all queries have been submitted to
the model.
> So there is absolutely no more lag that can be caused by client /
server
> communication. Again as in the initial example, once I've scrolled all
the way
> from top to bottom, scrolling is smooth.
> 
> This is particularly an issue on slower systems where the symptoms are
worse,
> but even on my quad core system this happens.
> 
> I don't think fetchMore and canFetchMore are going to be of any help
since by
> the time I do any scrolling all data has already been fetched.
> _______________________________________________
> Qt-interest mailing list
> Qt-interest at trolltech.com
> http://lists.trolltech.com/mailman/listinfo/qt-interest