On April 19, 2016 10:19:47 AM PDT, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: >On Tue, 19 Apr 2016 16:55:28 +0000 (UTC) >Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > >> ----- On Apr 19, 2016, at 10:34 AM, rostedt rostedt@xxxxxxxxxxx >wrote: >> >> > From: Steven Rostedt <rostedt@xxxxxxxxxxx> >> > >> > 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 Also, I understand there is one of these bitmaps per ring buffer, and the ring buffer is in the tens of megabytes. -- Sent from my Android device with K-9 Mail. Please excuse brevity and formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-trace-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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