Stewart Smith wrote: > On Wed, Aug 05, 2009 at 03:13:34PM +1000, Arjen Lentz wrote: >> The issue in MySQL has been overhead of such instrumentation, >> particularly also when not used. Some cause 5-20% perf loss which is >> unacceptable. > > 110% agree. > > If you're not doing analysis of anything, it shouldn't cost you. > > You also shouldn't have to restart, rebuild or anything like that. > > I think I know how to do this too. > > > I have this inkling that it's the "if(profiling_enabled)" inserted > everywhere that kills us. > > This is pretty easy to check. Say we have some function f() that is > going to do some counting for us (e.g. number of rows fetched, number of > times mutex X was taken). If profiling is disabled, we want this to use > 0 CPU. > > calling an empty function int f(int) a billion times in a loop is > roughly equivilnet of just running through the loop (yes, i built with > gcc -O0 and checked the produced code). By roughly I do mean next to > impossible to measure. > > If you add a simple "if(x) something;" to the function f(), it is > noticably slower! (roughly 20% in my tests). > > So we really don't want to do that compare. > > Now... about this time somebody is going to jump up and suggest using > DTrace to insert code at runtime. Not on Linux, so is worse than useless > here. > > But we can do some cool self modifying code tricks. > > The same do-nothing f() does not take any longer to run if we insert a few > no-ops. (i tried inserting 4 NOP instructions, which are single byte... > i do wonder if the multi-byte NOP instruction could help here too). > > So... when a profile hook is enabled, we just modify f() to call the > real profiling function. This can either be done with an atomic > instruction writing out the appropriate CALL instruction, or we can put > in a small JMP around the NOPs as we fill it out. > > > and there's a number of tricks to do this pretty easily for all the > possible points to hook in profiling stuff. >
I like this - which is odd, because I don't normally like things like this. But in this case.. ++ Monty _______________________________________________ Mailing list: https://launchpad.net/~drizzle-discuss Post to : [email protected] Unsubscribe : https://launchpad.net/~drizzle-discuss More help : https://help.launchpad.net/ListHelp
