Hi Ezequiel :) 2011/10/11 Ezequiel García <elezegarcia@yahoo.com.ar>:
Hi,
I have a SH4 board:
Alright, I use x86 for the basis of my (guessing) answer, so things might vary... :)
and suppose I have eaten pretty much all the memory, and have only 9 MB left (aprox.)
$ free total used free shared buffers cached Mem: 48072 42276 5796 0 172 3264 -/+ buffers/cache: 38840 9232 Swap: 0 0 0
so, excluding page cache aside, you had 9232 kilobytes = 9 MB free RAM.
ulimit -s 7500 I can now launch 10 threads. Each thread does not allocate anything, so it is only consuming the minimum overhead (aprox. 8 kb per thread, right?).
8MB failed, roughly 7.5 MB success....0.5 difference...
So, my question is:
Knowing that mmap doesnt actually consume the memory (due to overcommit=0, right?):
I guess this has somewhat a relation with OOM (Out of Memory detection). The thing is, even though mmap() doesn't allocate anything initially, but still you commit a memory area. And kernel, in this case, will promise up to certain degree that it will satisfy your request at its best. Initially, you requested 8 MB. Knowing that you had 9 MB free RAM, I think you got ENOMEM since that would just leave 1 MB and quite likely that is under safe minimum free RAM allowed. I roughly recall that somewhere around 2-5% of RAM must be kept unallocated from user need to cope with urgent situation or something like that. It was shown that, when you use 7.5 MB, that slight 0.5 MB made you didn't hit the mark...so you're safe now and kernel allow that. Sorry I couldn't point you to exact kernel source code line. lxr.linux.no is your best friend.... -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com