The first is something most users of database libraries will be familiar with: Bound variables. The idea is you prepare a query which is parsed into opcodes, and then at a later time you assign one of the
? opcode values to an actual integer or string. This is much faster as you do not have to re-parse the predicate, and also means you avoid failing in incomprehensible ways if the user searches for nonsense like
]@attr. Borrowing from SQL, the syntax should be familiar:
g_autoptr(XbQuery) query = xb_query_new (silo, "components/component/id[text()=?]/..", &error); xb_query_bind_str (query, 0, "gimp.desktop", &error);
The second feature makes the caller jump through some hoops, but hoops that make things faster: Indexed queries. As it might be apparent to some, libxmlb stores all the text in a big deduplicated string table after the tree structure is defined. That means if you do
then we only store just one string! When we actually set up an object to check a specific node for a predicate (for instance,
text()='fubar' we actually do
strcmp("fubar", "component") internally, which in most cases is very fast…
Unless you do it 10 million times…
Using indexed strings tells the
XbMachine processing the predicate to first check if
fubar exists in the string table, and if it doesn’t, the predicate can’t possibly match and is skipped. If it does exist, we know the integer position in the string table, and so when we compare the strings we can just check two
uint32_t’s which is quite a lot faster, especially on ARM for some reason. In the case of fwupd, it is searching for a specific GUID when returning hardware results. Using an indexed query takes the per-device query time from 3.17ms to about 0.33ms – which if you have a large number of connected updatable devices makes a big difference to the user experience. As using the indexed queries can have a negative impact and requires extra code it is probably only useful in a handful of cases. In case you do need this feature, this is the code you would use:
xb_silo_query_build_index (silo, "component/id", NULL, &error); // the cdata xb_silo_query_build_index (silo, "component", "type", &error); // the @type attr g_autoptr(XbNode) n = xb_silo_query_first (silo, "component/id[text()=$'test.firmware']", &error);
The indexing being denoted by
$'' rather than the normal pair of single quotes. If there is something more standard to denote this kind of thing, please let me know and I’ll switch to that instead.
The third feature is:
stem() XPath operator took a few minutes, but making it usable took up a whole weekend.
The query we wanted to run would be of the form
id[text()~=stem('?') but the
stem() would be called millions of times on the very same string for each comparison. To fix this, and to make other XPath operators faster I implemented an opcode rewriting optimisation pass to the
XbMachine parser. This means if you call
lower-case(text())==lower-case('GIMP.DESKTOP') we only call the UTF-8 strlower function
N+1 times, rather than
2N times. For
lower-case() the performance increase is slight, but for
stem it actually makes the feature usable in gnome-software. The opcode rewriting optimisation pass is kinda dumb in how it works (“lets try all combinations!”), but works with all of the registered methods, and makes all existing queries faster for almost free.
One common question I’ve had is if libxmlb is supposed to obsolete appstream-glib, and the answer is “it depends”. If you’re creating or building AppStream metadata, or performing any AppStream-specific validation then stick to the appstream-glib or appstream-builder libraries. If you just want to read AppStream metadata you can use either, but if you can stomach a binary blob of rewritten metadata stored somewhere, libxml is going to be a couple of orders of magnitude faster and use a ton less memory.
If you’re thinking of using libxml in your project send me an email and I’m happy to add more documentation where required. At the moment libxmlb does everything I need for fwupd and gnome-software and so apart from bugfixes I think it’s basically “done”, which should make my manager somewhat happier. Comments welcome.
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