叉()&内存分配行为 [英] fork () & memory allocation behavior

问题描述

我在禁用了交换功能且禁用了内存过量使用的系统上工作.

I work on a system in which swap is disabled and memory overcommit is disabled.

可以说我的进程当前消耗100 MB内存，而系统可用内存少于100 MB.

Lets say my process consumes 100 MB memory currently and the system free memory is less than 100 MB.

如果我执行fork()会失败，因为内核也尝试为子进程分配100 MB吗?

If I do a fork() will it fail because kernel tries to allocate 100 MB for the child process as well ?

我已阅读到Linux在分叉时使用写时复制，因此child&父级共享所有页面.所以我想fork应该成功吗?

I have read that Linux uses copy-on-write when forking, so child & parent share all the pages. So I guess fork should succeed ?

假设fork成功，可以说我在调用exec()之前在子进程中只有几行代码.因此，父级&子进程将继续共享文本段，并且除非子进程触及任何堆内存，否则内存使用量不应有任何变化.这是正确的吗?

Assuming fork succeeds, lets say I have few lines of code in the child process before calling exec(). So the parent & child will continue to share the text segment and there shouldn't be any change in memory usage unless child process touches any of the heap memory. Is this correct ?

Edit: One follow-up question: With swapping/overcommit disabled, can the cumulative VSS 
(Virtual Set Size) of all the processes be more than the available physical memory ?

I tried this out.  VSS of all the processes can be much more than that of the physical 
memory available.

pmap -x output from a process. 

total kB          132588   10744    7192

First number - Virtual Set Size
Second number - Resident Set Size
third number - dirty pages

RSS is < 10% of VSS. This is with swapping and overcommit disabled.

For every shared library loaded I see something like the following..

00007fae83f07000      32      24       0 r-x--  librt-2.12.so
00007fae83f0f000    2044       0       0 -----  librt-2.12.so
00007fae8410e000       8       8       8 rw---  librt-2.12.so

00007fae84312000     252     120       0 r-x--  libevent-2.0.so.5.0.1
00007fae84351000    2048       0       0 -----  libevent-2.0.so.5.0.1
00007fae84551000       8       8       8 rw---  libevent-2.0.so.5.0.1

I guess r-x segment is code and rw- is data. But there is a 2 MB segment 
that is not loaded. I see this 2 MB segment for every single shared library. 
My process loads a lot of shared libraries. That explains the huge difference 
between VSS & RSS.

Any idea what is that 2 MB segment per shared library ? 

When overcommit is disabled, if this process calls fork() will it fail when 
the free memory is less than VSS (132588 Kb) or RSS (10744) ?

推荐答案

是的，如果完全禁用内存过量使用，则fork将失败.它将失败，因为如果程序希望写入所有页面，它们可能会取消共享所有页面，而严格的过量提交模式将不允许这样做.

Yes, if memory overcommit is completely disabled then fork will fail. It will fail because the program might unshare all of its pages if it wished to write to them and the strict overcommit mode will not allow this.

您可以将fork替换为vfork，这将起作用. vfork设计为仅在fork/exec模型中与exec结合使用以启动新进程时使用.

You could replace fork with vfork and that would work. vfork is designed to be used only when combined with exec in the fork/exec model to launch a new process.

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