fork（）和内存分配行为
我在禁用交换并禁用内存过量使用的系统上工作。
假设我的进程当前消耗100 MB内存，系统可用内存小于100 MB。
如果我做了一个fork（），它会失败，因为内核试图为子进程分配100 MB？
我已经读过Linux在分叉时使用copy-on-write，所以child＆parent共享所有页面。 所以我猜叉子应该成功？
假设fork成功了，可以说我在调用exec（）之前在子进程中有几行代码。 所以父母和孩子将继续共享文本段，除非子进程触及任何堆内存，否则内存使用情况不应有任何变化。 它是否正确 ？
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) ?
是的，如果内存过量使用完全禁用，则fork将失败。 它会失败，因为如果程序希望写入它们，并且严格的overcommit模式不允许，程序可能会取消所有页面的共享。
你可以用vfork替换fork ，那就行了。 vfork的设计目的只有在与fork / exec模型中的exec结合起来才能启动新进程。
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