On Linux dladdr is slow. It does a linear search through a symbol table for each candidate library. It also has no memory so it plows through the same tables over and over again. The obvious solution is to not do that. :-) This is actually possible. Only the run-time location of a library is in doubt. Everything else is part of the ELF structure which can be read independently. All you really need is find is the base address of a candidate library and its file name. You just write that out and do the rest later. Now my_dladdr already exists. You just need the first part (with a few minor changes). This will speed things up but the true win is that you can access other symbol tables so you can potentially find static functions, etc. And yes, this approach does seem to work.
mostly blames to brendan.
Assignee: dougt → brendan
Tenthumbs, do you have a patch? If so, please attach. I'm giving this bug to bryner cuz he's been working on trace-malloc speedups recently; cc'ing dbaron, who is always up for trace-malloc fun. /be
Assignee: brendan → bryner
No patches yet, it too early. I've been working on my own malloc tool on and off for a while so I do know some things. Using a replacement dladdr speeds up things at least 5 times. Searching the symbol tables is really slow. We won't gain anything overall if we analyze the libraries the same way later on. The approach I'm beginning to like is to read an ELF executable, find the symbol tables, extract only the symbols of interest, merge all the symbols keeping track of where they came from etc., sorting the result by address, saving this derived data to disk, and reusing it as needed. There are a few quirks, though. Some addresses have multiple symbols. My libc has six names for fopen so picking the right name is a little tricky. Right now I'm contemplating how to arrange the derived data
Created attachment 88856 [details] example Here's an example with my own tool. It's one malloc call with a backtrace. The first line in each set of three is runtime data. The other two lines are derived from the ELF file. It's quite nice.
Created attachment 88857 [details] my_dladdr.c Here's a version of my_dladdr that uses documented features.
Status: NEW → RESOLVED
Last Resolved: 7 years ago
Resolution: --- → INCOMPLETE
You need to log in before you can comment on or make changes to this bug.