On Mon, 2015-02-02 at 13:22 -0800, Eric Anholt wrote: > Thomas Helland <thomashellan...@gmail.com> writes: > > > 2015-02-02 19:18 GMT+01:00 Jason Ekstrand <ja...@jlekstrand.net>: > >> Hi all, > >> I wanted to send out a quick message about naming conventions for mesa > >> entrypoints and dd table fallbacks. There has been some confusion and > >> disagreement about this with some of the stuff Laura has been doing to > >> implement DSA. Instead of side-track one of those patches for this > >> discussion, I thought it was worth its own e-mail. > >> > >> When Laura started working on DSA, I suggested that, since we're > >> refactoring > >> everything anyway, we refactor the guts of the entrypoint into a DSA-style > >> "internal entrypoint". This internal entrypoint takes a gl_context > >> pointer, > >> does less error checking, and actual mesa object pointers instead of GLuint > >> names. I'll get to why in a minute here. However, that leaves us needing > >> a > >> naming convention for three things: > >> > >> 1) The entrypoint itself (currently _mesa_EntryPoint) > >> 2) the internal entrypoint (Laura chose _mesa_entry_point) > >> 3) the software fallback for the DD table entry (Laura chose > >> _mesa_TableEntry_sw) > >> > >> I think we can all agree on not changing (1). Previously we had something > >> of a convention of _mesa_entry_point for (3) but it wasn't perfectly > >> consistent. Since we didn't have DSA, we didn't have anything for (2). > >> Personally, I'm OK with what Laura chose, but I understand that it's a > >> change of convention. Another option, would be to do _mesa_main_TableEntry > >> or _mesa_main_table_entry for the standard DD fallback. Or we can come up > >> with a different convention for the internal entry points. I don't care > >> much. > >> > >> Why the internal entrypoints? > >> > >> The first reason is that we have to do the refactor anyway, and that seems > >> reasonable. The more important reason is meta. This is something that > >> Kristian, Ken and I have talked about a good bit lately. I've done some > >> traces with meta lately and meta_begin/end show up a lot and what shows up > >> more is hash table lookups and, in particular, the mutex lock/unlock that's > >> involved. Inside meta, we do all sorts of stupidity like > > > > Kind of off-topic, but since you're mentioning overhead of hash tables: > > I've been doing some work on util/hash_table that is yet to be sent to the > > list. > > Thought I could share my experience if someone happens to be > > working on other improvements to the hash-tables. > > > > TLDR of approach: > > Power-of-two sized table instead of prime size. > > -> We can replace the modulo with bitmasking to fit keys in the table. > > -> Reduced overhead on insert, search, resize, etc. > > -> FNV-1a has good avalanche properties -> collisions is a no-issue. > > There was also the idea of using a much cheaper hash. The hash cost > itself is fairly large iirc. I'd love to see the two ideas tried out > side by side (a cheap crc32 should be easy to test).
Hi Thomas, I spent a bit of time trying to tweak the hash table back in November and posted my findings to the list [1]. One thing I found was using _mm_crc32_u8() to do the hash over the current FNV-1a implementation gave a nice bump in hashing performance and didn't see any noticeable change in collisions in my testing. The real problem I had was finding something cpu limited to test with. [1] http://lists.freedesktop.org/archives/mesa-dev/2014-November/071460.html _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
