On 11/7/14, 3:49 AM, Jean Delvare wrote:
> Hi Chet,
>
> Thanks for the fast reply and the explanations, very appreciated.
>
> On Thu, 06 Nov 2014 20:57:11 -0500, Chet Ramey wrote:
>> On 11/6/14 8:09 AM, Jean Delvare wrote:
>>> A memory leak has been reported in a bash script I maintain [1]. After
>>> investigation, I was able to shrink the test case down to:
>>>
>>> while true
>>> do
>>> sleep 1 &
>>> wait $!
>>> done
>>
>> This isn't a memory leak, and the memory use is bounded. The shell, as
>
> OK, this is in line with valgrind's claim that all allocated memory was
> still reachable. But how bounded is it? The shell script for which the
> issue was originally reported starts with about 3 MB of memory but
> reaches 32 MB over time (I don't know which version of bash did that
> though.) That seems to be a lot of memory just to record process exit
> statuses.
Maybe. That depends on the number of children your maxproc resource says
you can create. On my system, the struct that holds a pid's status is 16
bytes.
>> per Posix, keeps the statues of the last child_max processes. It gets
>> child_max from the process's resource limit (ulimit -v, 709 (?) on my
>> system). The list is FIFO, so when the number of background statuses
>> equals child_max, the oldest statuses are discarded.
>
> "help ulimit" says:
>
> -v the size of virtual memory
>
> the scope of which seems to exceed just the number of child process
> statuses to remember?
Sorry, it's `ulimit -u'.
> So I think in the case of a shell script daemon, I have two options:
> 1* Use "wait" without argument. As we are always waiting for a single
> process (sleep) to finish, I suppose this is equivalent to "wait
> $?", right?
Yes, as long as you mean $!.
> 2* Limit the value child_max. But using "ulimit -v" for that seems to
> be a rather violent and risky way to achieve this?
-u the maximum number of user processes
> Also three questions remain, if you would be kind enough to answer them.
>
> 1* Why is the memory consumption steady when using bash 3.0.16? Was
> this version of bash not recording process exit statuses yet?
That feature came in in bash-3.1.
> 2* If bash remembers the process statuses, how does one access the
> information? I couldn't find anything related to that in the manual
> page, but it's huge so maybe I missed it.
You can use wait with a pid argument, maybe one that you saved earlier
in a script or obtained from `ps'.
> 3* If I never call "wait" and thus never free the recorded exit
> statuses, does it mean that the system can't recycle the process IDs
> in question until the shell script exits? That would be a major flaw
> in my script :-(
No, the system will recycle the process ID as soon as bash reaps it with
waitpid(2). It's bash that is responsible for making sure to discard a
saved status if fork(2) returns a PID that's been saved.
Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU c...@case.edu http://cnswww.cns.cwru.edu/~chet/
