On Wednesday 03 June 2015 18:32:46 Milian Wolff wrote:
> Hey all,
>
> for a customer I looked into the performance of a model/view application
> with many (and I mean, many!) add/remove/modify operations on a model with
> a QSortFilterProxyModel on top. The obvious solution to speed things up is
> batching, which works nicely paired with layout{AboutToBe,}Changed. So far
> so good, but the issue that was now reported comes when one now *also*
> wants to enable MultiSelection in the view, and the user "accidentally"
> presses CTRL + A to select all items in the view. This then completely
> kills the performance, due to bottlenecks inside QAbstractItemModel, all
> related to QPersistentModelIndex.
>
> Some questions now from my side:
I forgot to mention: This targets Qt 4.7, but afaik all my findings below are
equally valid to current Qt 5 code.
> a) Why is the hash of indexes in QAbstractItemModelPrivate::Persistent not
> unique - i.e. why is insertMulti required? I ask, b/c the "it + 1" operation
> in QAbstractItemModelPrivate::Persistent::insertMultiAtEnd is _extremely_
> slow for large lists of persistent model indices, due to the cache misses
> etc. pp. involved. The documentation says:
>
> "There should be only one instance QPersistentModelIndexData per index,
> but in some intermediate state there may be severals of PersistantModelIndex
> pointing to the same index, ..."
>
> What intermediate state is that? When I look at the uses of this function,
> they always use the pattern
>
> persistent.indexes.erase(...);
> ...
> persistent.insertMultiAtEnd(...);
>
> I fail to see how/where this intermediate state can occur.
I just recompiled Qt with this patch applied:
diff --git a/src/corelib/kernel/qabstractitemmodel.cpp
b/src/corelib/kernel/qabstractitemmodel.cpp
index 1eb709c..ec7006b 100644
--- a/src/corelib/kernel/qabstractitemmodel.cpp
+++ b/src/corelib/kernel/qabstractitemmodel.cpp
@@ -3464,8 +3464,10 @@ bool QAbstractListModel::dropMimeData(const QMimeData
*data, Qt::DropAction acti
*/
void QAbstractItemModelPrivate::Persistent::insertMultiAtEnd(const
QModelIndex& key, QPersistentModelIndexData *data)
{
+ Q_ASSERT(indexes.count(key) < 2);
QHash<QModelIndex,QPersistentModelIndexData *>::iterator newIt =
- indexes.insertMulti(key, data);
+ indexes.insert(key, data);
+ return;
QHash<QModelIndex,QPersistentModelIndexData *>::iterator it = newIt + 1;
while (it != indexes.end() && it.key() == key) {
qSwap(*newIt,*it);
with a developer build, and assertions enabled, everything still works as
expected (just much faster). I ran the following unit tests, all still pass,
or behave just as without this patch:
qabstractitemmodel
qabstractproxymodel
qsortfilterproxymodel
qtreeview
qlistview
My own test app also works without asserting. So again - why the multiHash -
obsolete? If so, I'll push the required changes to Qt 4 and Qt 5 to remove
QAbstractItemModelPrivate::Persistent::insertMultiAtEnd.
Bye
--
Milian Wolff | milian.wo...@kdab.com | Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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