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The question is: can you successfully apply for 8g memory on a machine with 4GB physical memory?

2022-07-22 05:56:00 【InfoQ】

Hello everyone , I'm Xiao Lin .
In the morning, I saw readers discussing these interview questions in the group ：
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among , The first question is 「
stay  4GB  On a machine with physical memory , apply  8G  What happens to memory ？
」 There is a big controversy , Some people say that they will fail to apply , Some people say you can apply successfully .
This problem has no preconditions , Just say the answer is playing hooligans . Because in  32  Bit operating system and  64  Bit operating system scenario , The answer is different .
in addition , We still have to wait until the application is completed  8G  Memory will not be used after , Will be used is a situation , Not being used is another case .
therefore , We'll discuss it in different scenarios .

Text

Application through  malloc  When the function applies for memory , In fact, the application is virtual memory , Physical memory is not allocated at this time .
When the application reads and writes this virtual memory ,CPU  Will access this virtual memory ,  At this time, you will find that the virtual memory is not mapped to the physical memory , CPU  It will produce
Missing pages interruption
, The process will switch from user mode to kernel mode , And give the page missing interrupt to the kernel  Page Fault Handler （ Missing page interrupt function ） Handle .
The missing page interrupt handler will see if there is free physical memory ：
  • If there is , Just allocate physical memory directly , And establish the mapping relationship between virtual memory and physical memory .
  • If there is no free physical memory , Then the kernel will begin to reclaim memory , If the memory recycling work is finished , The free physical memory is still unable to meet the physical memory application , Then the kernel will trigger the last big move  OOM （Out of Memory） Mechanism .
32  Bit operating system and  64  The size of virtual address space of bit operating system is different , stay  Linux  Operating system , The interior of the virtual address space is divided into
Kernel space and user space
Two parts , As shown below ：
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It can be seen from here that ：
  • 32  The kernel space of bit system takes up  1G, At the top , The rest  3G  It's user space ;
  • 64  The kernel space and user space of a bit system are both  128T, Occupy the highest and lowest memory space respectively , The rest of the middle is undefined .
Now you can answer this question ： stay  32  Bit operating system 、4GB  On a machine with physical memory , apply  8GB  Memory , What will happen? ？
because  32  Bit operating system , The process can only apply for at most  3 GB  Size of virtual memory space , So the process application  8GB  Memory , It will fail during the virtual memory request phase （ I don't have  32  Bit operating system test , I guess the reason for the failure is  OOM）.
stay  64  Bit operating system 、4GB  On a machine with physical memory , apply  8G  Memory , What will happen? ？
64  Bit operating system , The process can use  128 TB  Size of virtual memory space , So the process application  8GB  Memory is OK , Because the process is applying for virtual memory , As long as you don't read or write this virtual memory , The operating system will not allocate physical memory .
We can do a simple test , My server is  64  Bit operating system , But the physical memory is only  2 GB.
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Now? , I'm on the machine , apply  4 GB  Memory , Note that the following code simply allocates virtual memory , Not using virtual memory ：
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>

int main() {
 int ret;
 char* addr[4];
 printf(&quot; Use cat /proc/%d/maps View memory allocation \n&quot;,getpid());
 size_t s = 1024 * 1024 * 1024;
 int i = 0;
 for(i = 0; i < 4; ++i) {
 printf(&quot;alloc size = %d\n&quot;, s);
 addr[i] = (char*) malloc(s);
 printf(&quot; Main thread call malloc after , apply 1gb Memory size , This memory start address ：0X%x\n&quot;, addr[i]);
 }
 getchar();
 return 0;
}
Then run the code , You can see ,
Although my physical memory is only  2GB, But the program is normally allocated  4GB  Size of virtual memory
：
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We can check the virtual memory size of the process through the following command ：
# ps aux | grep alloc_4g
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 7797 0.0 0.0 4198540 352 pts/1 S+ 16:58 0:00 ./alloc_4g
among ,VSZ  Represents the size of virtual memory used by the process ,RSS  Represents the physical memory used by the process . You can see ,VSZ  The size is  4198540, That is to say  4GB  Virtual memory .
then , Let's change the code , After applying for virtual memory , adopt  memset  Function uses this virtual memory , See what happens .
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>

int main() {
 int ret;
 char* addr[4];
 printf(&quot; Use cat /proc/%d/maps View memory allocation \n&quot;,getpid());
 size_t s = 1024 * 1024 * 1024;
 int i = 0;
 for(i = 0; i < 4; ++i) {
 printf(&quot;alloc size = %d\n&quot;, s);
 addr[i] = (char*) malloc(s);
 printf(&quot; Main thread call malloc after , apply 1gb Memory size , This memory start address ：0X%x\n&quot;, addr[i]);
 // Accessing virtual memory
 memset(addr[i], 0, s);
 }
 getchar();
 return 0;
}
Running results ：
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You can see ,
I'm applying  2GB  After virtual memory , Then I immediately used this virtual memory , Because the physical memory of this machine is only  2 GB, So it happens  OOM
.
thus ,  Verification is complete . In a brief summary ：
  • stay  32  Bit operating system , Because the process can only apply for  3 GB  Size of virtual memory , So apply directly  8G  Memory , The application will fail .
  • stay  64 position   Bit operating system , Because the process can only apply for  128 TB  Size of virtual memory , Even if the physical memory is only  4GB, apply  8G  Memory is no problem , Because the requested memory is virtual memory , When this virtual memory is accessed , Because there is not enough physical space , It will happen  OOM.
Okay , The water is out ！
Link to the original text ：https://mp.weixin.qq.com/s/LbtXW1NtG6VqEpsqqOL3Ig
author ： Kobayashi coding
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