On Tue, 19 Apr 2016 16:55:28 +0000 (UTC) Mathieu Desnoyers <[email protected]> wrote:
> ----- On Apr 19, 2016, at 10:34 AM, rostedt [email protected] wrote: > > > From: Steven Rostedt <[email protected]> > > > > In order to add the ability to let tasks that are filtered by the events > > have their children also be traced on fork (and then not traced on exit), > > convert the array into a pid bitmask. Most of the time the number of pids is > > only 32768 pids or a 4k bitmask, which is the same size as the default list > > currently is, and that list could grow if more pids are listed. > > > > This also greatly simplifies the code. > > The maximum PID number can be increased with sysctl. > > See "pid_max" in Documentation/sysctl/kernel.txt > > What happens when you have a very large pid_max set ? I discussed this with HPA, and it appears that the pid_max max would require a bitmap of about 1/2 meg (the current default is 8k). This is also why I chose to keep the bitmap as vmalloc and not a continuous page allocation. > > You say "most of the time" as if this was a fast-path vs a slow-path, > but it is not the case here. I meant "most of the time" as "default". Yes, you can make the pid_max really big, but in that case you better have enough memory in your system to handle that many threads. Thus a 1/2 meg used for tracking pids shouldn't be an issue. > > This is a configuration option that can significantly hurt memory usage > in configurations using a large pid_max. No, it is created dynamically. If you never write anything into the set_event_pid file, then you have nothing to worry about, as nothing is allocated. It creates the array when a pid is added to the file, and only then. If it fails to allocate, the write will return -ENOMEM as the errno. Again, if you have a large pid_max your box had better have a lot of memory to begin with, because this array will be negligible compared to the memory required to handle large number of tasks. > > FWIW, I implement a similar feature with a hash table in lttng-modules. > I don't have the child process tracking though, which is a neat improvement. I originally had a complex hash algorithm because I too was worried about the size of pid_max and using a bitmap, but HPA convinced me it was the way to go. -- Steve
