[Mono-dev] Size of thread in Mono (65MB per thread?)

Nicolas Antoniazzi nicolas.antoniazzi at gmail.com
Fri Dec 20 09:48:41 UTC 2013

Ok, I understand better the underline mechanism. I will try change my
virtualization system with cgroups to simulate limitation of physical

But in the meantime, I am curious to know what is the need for a thread to
reserve 64MB of virtual memory. I understand a need of 1 or 2MB for its
stack plus few other kilos. But 64MB seems a lot to me :)

Thanks a lot for your help!

2013/12/20 Marek Habersack <grendel at twistedcode.net>

> On 12/20/2013 09:10 , Nicolas Antoniazzi wrote:
>> I am not using OpenVZ but a solution that we developed based on Linux
>> kernel calls because we really need to bootstrap a
>> virtual environment in less than 50ms.
>> I tested the same program on a .Net platform and after 1000 threads
>> created, the whole application used 48MB of RAM. It
>> sounds really strange to me that a Thread, that in theory should be a
>> light process, takes 65MB of virtual memory.
>> In the meantime, I am not expert in differences between virtual and
>> physical memory, but, does your answer mean that if
>> mono would detects that my system only has 500MB of physical memory, it
>> would reserve less amount of memory per thread?
> Unix systems work based on a bit different principle than the Windows
> ones. Namely, as Nikita mentions in his other mail, the virtual memory is
> nearly free of any limits - your process reserves the right to use that
> much memory, but it doesn't actually use ("commit") it physically. The
> virtual memory reservation is merely a hint of what can be consumed by the
> program, should it need it. You can observe that by running the following
> command on Linux:
>   ps axu|less
> now look at the headers at the top of the display:
> root      1203  0.0  0.1   3436  1140 ?        S    Nov30   0:00
> upstart-udev-bridge --daemon
> root      1209  0.0  0.1   9552  1416 ?        Ss   Nov30   0:00
> /lib/systemd/systemd-udevd --daemon
> The VSZ and RSZ columns represent, respectively, the virtual (reserved)
> and real (committed) memory - you always want to look at the latter for
> actual memory usage of a process. Now, in my opinion, any virtualization
> system that limits the virtual memory is inherently flawed (at least on
> Linux) and this is because of the RSS vs VSZ difference I explained above
> but also because of another detail. Namely, each thread, as you know, is
> not a separate process per se (even though it's got its own PID) but rather
> a compartment in your parent process which has its own stack, own CPU state
> etc but it *still* lives in the same memory space and share the memory
> allocation with the parent. So if you look at the thread's memory usage
> figures you will get exactly the same numbers as for the main process - but
> it does not mean the each thread is using that much memory. In fact, memory
> is not allocated to the thread but to the process only.
> Therefore, if your virtualization software looks at each PID's virtual
> memory usage and imposes a limit on that figure, then certainly, you will
> get an OOM in no time - even though the physical memory usage will be
> actually much lower.
> hope that helps a bit,
> marek
>> Maybe there is a way to send some parameters to mono or to change some
>> content in /proc to simulate a smaller amount of
>> physical memory?
>> Thanks for your answer!
>> 2013/12/20 Nikita Tsukanov <keks9n at gmail.com <mailto:keks9n at gmail.com>>
>>     Don't use OpenVZ, it limits _virtual_ memory, not physical. Mono
>> threads use a small amount of physical memory, but
>>     might reserve high of virtual memory space. You'd rather try KVM/Xen
>> virtualization.
>>     Regards,
>>     Nikita
>>     2013/12/19 Nicolas Antoniazzi <nicolas.antoniazzi at gmail.com <mailto:
>> nicolas.antoniazzi at gmail.com>>
>>         Hi,
>>         I am using Mono in a virtualized environment with 512MB of RAM.
>>         I made a very simple program which starts 10 threads in a loop
>> and apparently, every time that I start a new
>>         thread, approximately 65MB of memory is used.
>>         In my case, I can run 5 threads, but for the 6th, the program
>> crashes (without any exception). 150MB are already
>>         consumed without the use of any thread.
>>         Is it a normal behavior?
>>         P.S: Sorry I double posted this message on mono-list because I
>> did not understood that third party programmers
>>         also had to come on this devel list.
>>         Thanks!
>>         --
>>         Nicolas Antoniazzi
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