[ale] Allocation of memory

[Phil Turmel]

Also, see pthread_attr_setstacksize()

On 01/04/2014 01:01 AM, alan at alanlee.org wrote:
> 
> pthread_create will setup a new VMA (virtual memory area) for the stack frame
> for the new thread, however most if not all of the page frames in that area will
> point to null.  As the new thread executes and uses more and more of the stack,
> those null page references will fault and Linux will on-demand map a free page
> into that frame.  When that VMA is setup, Linux will, in addition, subtract the
> number of page frames in that area from the total available pages in the system
> (physical ram + swap space / page size).  This is so that if all memory users in
> the system suddenly tried to use all of their pages, the system doesn't run
> short.  You just have to be aware unless you lock those page frames, Linux is
> free at any time to take any page and either swap it out or completely throw it
> out of ram (if there is a file backing store like .text pages).
> 
> -Alan
> 
> 
> On January 4, 2014 at 12:25 AM Alex Carver <agcarver+ale at acarver.net> wrote:
>> Ok, I think I'm a bit fogged due to vacation (sure, that's the ticket)
>> so help me clear something up as I fiddle with a little coding project.
>>
>> After reading and rereading and rerereading, the Linux Way (TM) of most
>> memory allocation for things like pthreads and the like is to ask for a
>> bunch of space and the OS will pseudo-grant that request with the caveat
>> that anything that isn't really and truly used can get swiped away for
>> something else.
>>
>> By example, a little program I'm writing spawns a thread with each new
>> network connection using pthread_create. I'm watching the processes
>> with top while testing (I'm trying to keep the program small and lean
>> but still handle multiple operations). When it first starts up and no
>> threads have been spawned, I see a resident size and a data (stack) size
>> of some amount of kB. When a thread is spawned, the resident size
>> increases slightly but the data-stack size jumps by about 8 MB per
>> thread (the default request for pthreads on Linux). What I'm reading
>> says that the 8 MB isn't really 8 MB, it's just a request and probably
>> doesn't use all that much so the rest of the requested RAM that is
>> unused while the entire program is running could potentially go off to
>> another application. Have I read that right? When the threads close
>> down and all that's left is the main process, the data-stack amount
>> doesn't drop back down, it just sits there.
>>
>> I'm asking because I'm trying to make sure this program has a small
>> memory footprint to start (which for the most part it does, resident
>> size is under 600 kB and the initial data-stack size is only 300 kB) but
>> I'll eventually run this program on a small machine so I don't want 8 MB
>> to just vanish every time a thread starts up. It won't be many threads,
>> maybe 10 at the most but still, I want to try to use the least memory
>> possible just for fun. :)
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