Re: Problem with mlockall() and Threads: memory usage

[Terry Barnaby <terry1@beam.ltd.uk>]

Hi David,

We do want improved latency, but with reasonable memory usage. This is
a soft real-time system. At the moment the memory usage is far too high in
our application.

With 20 threads runing the system will lock 160MBytes of memory just for stack
space (8 MBytes each), although the application probably only needs 2MByte in
total.
We can reduce the maximum stack size per thread, but then if a thread
increases its stack size beyond this the application will crash with a
segment fault, not good ...

For our use, mapping in physical memory as required for a growing stack would be
a good compromise between latency and memory usage. Once the system has run the
worker threads for a short time all of the needed stack memory will be locked in
and latency will be controlled. If a thread needs more memory for stack in a particular
instance, there will be a latency hit but this would be acceptable and
much better than a crash.

Terry

David Schwartz wrote:
>>Thanks for that.
>>I have done some more investigating, and on my system (Standard RedHat 9)
>>the stack ulimit is set to 8192 KBytes. So it appears that the thread
>>library/kernel threads pre-allocates, and writes to, 8129 KBytes
>>of stack per
>>thread and so then mlockall() locks all of this in memory.
>>
>>Should'nt the Thread library grow the stack rather than
>>preallocate it all even
>>with mlockall() like malloc ?
> 
> 
> 	I thought you wanted improved latency. Surely having to find a page for you
> when your stack grows will add unpredictable latency. So, no, the thread
> library should reserve the stack when 'mlockall(MCL_FUTURE)' is specified.
> 
> 	I do agree that having an 'initial stack size' in additional to a 'maximum
> stack size' would be a good idea. The former good for application that are
> concerned about physical memory usage and the latter for applications
> concerned about virtual memory usage.
> 
> 	DS
> 
> 
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-- 
Dr Terry Barnaby                     BEAM Ltd
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