Hi, On Tue, Oct 19, 2010 at 7:15 PM, Joel Rosdahl <j...@rosdahl.net> wrote: > On 2010-10-18 03:44, Justin Lebar wrote: >> So it appears that 13% of my CPU time is spent computing md4 hashes, >> while another 25% is spent in hash_source_code_string but outside the >> MD4 code. >> >> To someone new to the code like me, it appears that there's some room >> for optimization here. > > Indeed. (This was by the way mentioned on the list a couple of weeks > ago: http://email@example.com/msg00532.html) [...] >> * Why does ccache still use MD4? Surely there's a better / faster hash >> out there. I noticed that ccache includes murmurhash, but it doesn't >> seem like it's used in too many places. There's probably a good reason >> for this, but it's not too apparent to me. >> [...] > > I added murmurhash to get a good hash function for some hash tables I > introduced when implementing the direct mode, which is a relatively late > invention. I'm far from an expert on hash functions, but surely > murmurhash and similar functions (that are designed to be good hash > functions for a hash table) aren't suitable to use for identifying > (without verification) arbitrary content like a cryptograhic hash > function is. Even the 64-bit version of murmurhash has way too high > collision rate. > > MD4 has been there from the start and neither Tridge or I have seen any > reason to switch it. MD5, SHA1 and other even more modern cryptograhic > hash functions are indeed stronger but also slower, and the increased > resistance against various crypto attacks doesn't seem necessary in a > tool like ccache. That said, I'm sure there nowadays may exist hash > functions that are both better (i.e., with lower collision rate) AND > faster than MD4. Do you (or anyone else) know of any with properties > that would be a good fit for ccache?
I'm CC'ing a couple of gurus from FFmpeg in the hope that they can help us. _______________________________________________ ccache mailing list firstname.lastname@example.org https://lists.samba.org/mailman/listinfo/ccache