plugins,caches, startup time

[Aleix Pol]

On Sat, Oct 1, 2016 at 12:18 AM, Sebastian Kügler <sebas at kde.org> wrote:
> On Friday, September 30, 2016 4:23:52 PM UTC Aleix Pol wrote:
>> > Nice, I suppose KPackages are a little different to plugins, since there's
>> > more overhead reading, the package metadata is stored as .desktop file,
>> > which means more conversion costs compared to the binary json that's in
>> > the plugin binaries. Aleix has recently been looking into converting it
>> > to json, but that would only save the .desktop to json part, binary json
>> > is still much quicker to read (the format is pretty close to Qt's
>> > internal representation in memory, deserialization is almost entirely
>> > I/O-bound.
>>
>> It's not only about loading from json. Keep in mind that even adding a
>> cache we'll be loading each metadata.desktop a number of times either
>> way (both using KConfig parsing and KCoreAddons' DesktopFileParser).
>
> No we won't, the caching code stores/restores a binary json file,
> deserialization of that is really cheap, the KPluginMetaData is created from
> the json object, which is the cheapest constructor you could think of (it
> neither has to go through DesktopFileParser nor JSON deserialization, see
> kcoreaddons[sebas/kpluginindex2]/src/lib/plugin/kpluginloader.cpp:283
>
> When *creating* the cache, yes, we have to go through DesktopFileParser or,
> once we've migrated everything to metadata.json, through Qt's JSON
> deserialization. The cache will be faster to read in both cases.

As I pointed out in this RR [1], that's not entirely the case, because
to create an applet we pass the path instead of the metadata tuple
(which parses each metadata.desktop file twice).

It will have an impact when it comes to listing plugins, but it mostly
has an impact because we're in fact caching runs of the
DesktopFileParser which is quite inefficient.
Furthermore, there's little reason to make tens of listing calls in
matter of few seconds and not cache these in-process.

Aleix

[1] https://git.reviewboard.kde.org/r/129057/