0746df4906
GData tracks the entries in a linear array, and performs a linear
search. That is very good, as long as the number of entries is small.
However, when it grows, performance gets worse. It's not clear what the
exact threshold is, or what the recommended maximum is.
Also, GData is used to attach arbitrary user data to GObject. Which is a
great feature, that a user might want to use. In that case, they may be
less concerned about performance to get the benefits of the feature, and
thus add more data than is best (though, it's unclear how much is too
much).
Also, GObject adds entries in its qdata. Hence, also for basic GObject
operations, it's important that this performs well. Note that access to
GData happens while holding a lock, we want to minimize the time while
holding that lock.
This patch ensures that access to GData is O(1) (and reasonably fast).
It thus allows to use GData in ways that wasn't advised previously.
There are alternatives, like using a binary search tree or always use a GHashTable.
Instead, we keep the linear buffer. Only when the buffer grows to
ALLOC_THRESHOLD_INDEX entries, we will start generating and maintaining
a GHashTable index. So the common case where objects have few entries
does not change. The memory overhead is only there, when the list grows.
ALLOC_THRESHOLD_INDEX is set to 64 entries. We will allocate such a
large buffer when adding the 33rd entry. During shrink, we will drop the
buffer again when shrinking down to 16 entries.
The reason for not always using the GHashTable is to save the memory in
common cases where there are few entries.
We use g_hash_table_add() to exploit the GHashTable optimization. We
also let it point to the GDataElt from the linear array. The benefit is
that we don't require individual allocations. The downside is that
during reallocation we need to regenerate the entire index.
In my tests this actually performs well. For example, following are
timings for calling g_dataset_id_get_data() in a loop. This is the
lookup time for an entry that doesn't exist. Obviously, the linear
search would scale always best when we would only lookup the first entry
in the list.
num-entries time-before time-after
1 0.144 0.145
2 0.146 0.146
5 0.149 0.150
10 0.155 0.157
20 0.172 0.170
32 0.248 0.254
33 0.249 0.184 <== index in use
40 0.284 0.183
50 0.317 0.189
75 0.370 0.184
100 0.442 0.183
300 1.044 0.186
1000 3.170 0.184
10000 31.597 0.189
GLib
GLib is the low-level core library that forms the basis for projects such as GTK and GNOME. It provides data structure handling for C, portability wrappers, and interfaces for such runtime functionality as an event loop, threads, dynamic loading, and an object system.
The official download locations are: https://download.gnome.org/sources/glib
The official web site is: https://www.gtk.org/
Installation
See the file ‘INSTALL.md’. There is separate and more in-depth documentation for building GLib on Windows.
Supported versions
Upstream GLib only supports the most recent stable release series, the previous stable release series, and the current development release series. All older versions are not supported upstream and may contain bugs, some of which may be exploitable security vulnerabilities.
See SECURITY.md for more details.
Documentation
API documentation is available online for GLib for the:
Discussion
If you have a question about how to use GLib, seek help on GNOME’s Discourse
instance. Alternatively, ask a question
on StackOverflow and tag it glib.
Reporting bugs
Bugs should be reported to the GNOME issue tracking system. You will need to create an account for yourself. You may also submit bugs by e-mail (without an account) by e-mailing incoming+gnome-glib-658-issue-@gitlab.gnome.org, but this will give you a degraded experience.
Bugs are for reporting problems in GLib itself, not for asking questions about how to use it. To ask questions, use one of our discussion forums.
In bug reports please include:
- Information about your system. For instance:
- What operating system and version
- For Linux, what version of the C library
- And anything else you think is relevant.
- How to reproduce the bug.
- If you can reproduce it with one of the test programs that are built
in the
tests/subdirectory, that will be most convenient. Otherwise, please include a short test program that exhibits the behavior. As a last resort, you can also provide a pointer to a larger piece of software that can be downloaded.
- If you can reproduce it with one of the test programs that are built
in the
- If the bug was a crash, the exact text that was printed out when the crash occurred.
- Further information such as stack traces may be useful, but is not necessary.
Contributing to GLib
Please follow the contribution guide to know how to start contributing to GLib.
Patches should be submitted as merge requests to gitlab.gnome.org. Note that you will need to be logged in to the site to use this page. If the patch fixes an existing issue, please refer to the issue in your commit message with the following notation (for issue 123):
Closes: #123
Otherwise, create a new merge request that introduces the change. Filing a separate issue is not required.