On Fri, Nov 02, 2018 at 11:30:03PM -0700, David Miller wrote: > From: David Miller <da...@davemloft.net> > Date: Wed, 31 Oct 2018 09:08:16 -0700 (PDT) > > > From: Jiri Olsa <jo...@redhat.com> > > Date: Wed, 31 Oct 2018 16:39:07 +0100 > > > >> it'd be great to make hist processing faster, but is your main target here > >> to get the load out of the reader thread, so we dont lose events during the > >> hist processing? > >> > >> we could queue events directly from reader thread into another thread and > >> keep it (the reader thread) free of processing, focusing only on event > >> reading/passing > > > > Indeed, we could create threads that take samples from the thread processing > > the ring buffers, and insert them into the histogram. > > So I played around with some ideas like this and ran into some dead ends. > > I ran each mmap ring's processing in a separate thread. > > This doesn't help at all, the problem is that all the threads serialize > at the pthread lock for the histogram part of the work. > > And the histogram part dominates the cost of processing each sample.
yep, it suck.. I was thinking of keeping separate hist objects for each thread and merge them at the end > > Nevertheless I started work on formally threading all of the code that > the mmap threads operate on, such as symbol processing etc. and while > doing so I came to the conclusion that pushing the histogram processing > only to a separate thread poses it's own set of big challenges. > > To make this work we would have to make a piece of transient on-stack > state (the processed event) into allocated persistent state. > > These persistent event structures get queued up to the histogram > thread(s). > > Therefore, if the histogram thread(s) can't keep up (and as per my > experiment above, it is easy to enter this state because the histogram > code itself is going to run linearly with the histgram lock held), > this persistent event memory will just get larger and larger. > > We would have to find some way to parallelize the histgram code to > make any kind of threading worthwhile. do you have some code I could check on? I'm going to make that separate thread to get the processing out of the reading thread.. I think we need that in any case, so the ring buffer is kept free as fast as possible thanks, jirka
