linux-kernel - Re: Patch kernel: I have 8 Gbytes RAM, but why I can only allocate 2.8 Gbytes RAM for a single process?

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Date:	Mon, 25 Feb 2008 18:22:13 +0100
From:	"J.C. Pizarro" <jcpiza@...il.com>
To:	"Ady Wicaksono" <ady.wicaksono@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Patch kernel: I have 8 Gbytes RAM, but why I can only allocate 2.8 Gbytes RAM for a single process?

2008/2/25, Ady Wicaksono <ady.wicaksono@...il.com>:
> I have 8 Gbytes RAM, but why I can allocate 2.8 Gbytes RAM for a single process?
>  How to patch kernel so I have more than 2.8 Gbytes limitation?
>
>  Kernel:
>  ---------------------------
>  Linux xxx.com 2.6.9-023stab046.2-enterprise #1 SMP Mon Dec 10 15:22:33
>  MSK 2007 i686 i686 i386 GNU/Linux
>
>  Mem:
>  ---------------------------
>  # cat /proc/meminfo
>  MemTotal:      8296484 kB
>  MemFree:         50416 kB
>  Buffers:         64412 kB
>  Cached:        4927328 kB
>  SwapCached:          0 kB
>  Active:        6710828 kB
>  Inactive:      1065384 kB
>  HighTotal:     4980736 kB
>  HighFree:         1024 kB
>  LowTotal:      3315748 kB
>  LowFree:         49392 kB
>  SwapTotal:    10256376 kB
>  SwapFree:     10255732 kB
>  Dirty:              64 kB
>  Writeback:           0 kB
>  Mapped:        3054960 kB
>  Slab:           393224 kB
>  CommitLimit:  14404616 kB
>  Committed_AS:  6318152 kB
>  PageTables:      34892 kB
>  VmallocTotal:   303096 kB
>  VmallocUsed:     22360 kB
>  VmallocChunk:   280496 kB
>
>
>  CPU (8 processor id from 0-7), one of them is:
>  ---------------------------
>  processor       : 0
>  vendor_id       : GenuineIntel
>  cpu family      : 15
>  model           : 6
>  model name      : Intel(R) Xeon(TM) CPU 3.00GHz
>  stepping        : 4
>  cpu MHz         : 2993.054
>  cache size      : 2048 KB
>  physical id     : 0
>  siblings        : 4
>  core id         : 0
>  cpu cores       : 2
>  fdiv_bug        : no
>  hlt_bug         : no
>  f00f_bug        : no
>  coma_bug        : no
>  fpu             : yes
>  fpu_exception   : yes
>  cpuid level     : 6
>  wp              : yes
>  flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
>  mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx
>  lm constant_tsc pni monitor ds_cpl est cid xtpr
>  bogomips        : 5989.55
>
>  App to test memory limit:
>  ---------------------------
>  #include <stdio.h>
>  #include <stdlib.h>
>
>  int main(){
>         size_t siz = 100 * 1024 * 1024 ;
>         size_t idx = 1 ;
>         void *ptr;
>
>         for (;;){
>                 ptr = malloc ( siz * idx );
>                 if(!ptr)
>                         break ;
>                 free(ptr);
>                 idx++;
>         }
>         printf ("Max malloc %d * 100 MB \n", idx - 1 );
>         return (0);
>  }
>
>  App result: Max malloc 28 * 100 MB ==> 2.8 Gbytes

1. It's a 32-bit processor Xeon with 8 GiB of RAM. OK?
2. The 32-bit userspace's process is always limited to <3.0 GiB ( < 0xC0000000 )
3. Enable PAE (64 GB option in the kernel) to address the 8 GiB of RAM
using PAE,
    also you can have many processes of ~3 GiB per process.
    I'm not sure if the PAE's three-level paging works efficient in linux.

   ;)
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