Just to make sure I have this correct, an application can call brk to
allocate memory from its heap space for upto 1GB, after which it has to
switch to using mmap for the rest. Is this 1GB fixed or will it ever
change? (if it does, it might break my application).
>around 900M if the malloc uses sbrk, but more if it uses mmap.
>modern libc's will use mmap for large allocations, since unlike
>sbrk, mmap can actually free memory back to the OS.
Which version of libc has malloc that uses mmap after 1GB?
>lots of trivial-sized allocations are a really bad idea in any case.
>doing it in 1M chunks will give you more like 3G.
But what if I have an application that doesent know how much memory it will
need? My application is reading in a file, parsing it and constructing a
huge in-memory database. Should I write my own library that will first mmap
a large (say 2 GB) memory and then dish out smaller chunks to the
application from there on an as need babsis? This is what malloc should
really do. What is the general approach taken for these kind of
applications?
> the issue is
> actually that the kernel normally permits 3G of VM to each task,
> but starts mmaped allocations at the 1G mark. that limits the sbrk
It seems complicated for an application that wants to use a large amount of
memory to know that it first has to use brk then switch to mmap after a
1GB... or am I understanding this incorrectly? It also might break
portability, right?
> heap, of course. but it's easy and not harmfull to shift the
> starting point for mmap if you must use tiny mallocs. a better
How do I do this? Is this a kernel change?
> solution would probably be to have mmaped areas start at 3G and
> grow down, but that's not in the kernel yet.
> regards, mark hahn.
Thank you all very much :)
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