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Re: Understanding the working of Shared memory using mmap()

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From	Rainer Weikusat <rweikusat@talktalk.net>
Newsgroups	comp.os.linux.development.system
Subject	Re: Understanding the working of Shared memory using mmap()
Date	Mon, 13 Mar 2023 17:35:08 +0000
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Pavankumar S V <pavankumarsv96@gmail.com> writes:
> As per my understanding, I can explain the working of mmap() briefly
> like this: When a process(let's call it process1) calls mmap on a
> regular file, that file is first copied to the page cache. Then the
> region of page cache which contains the file is mapped to virtual
> address space of the process1(This memory region is called
> memory-mapped file).  If another process(let's call it process2) calls
> mmap on the same file, then the same page cache that was mapped to
> process1 will get mapped to the virtual address space of process2.
> When the processes wants to access the file, they simply access this
> memory mapped file which is very faster. Also the data modified by
> process1 can be seen by process2.

It *may* be faster. But address space manipulations, page faults
occurring while populating some part of the virtual address space of a
process and cache- and TLB-misses are all expensive operations, hence,
it may well be not.

> I have a query here. Please clarify it:
> When the process1 wants to write some data to the file, it will write
> to this memory mapped file. Then these dirty pages that are private to
> the process1 should be copied to the page cache. When will the kernel
> do this copying to page cache and how frequently?

Not at all. If the mapping is done as MAP_PRIVATE, the process will gets
its own copy of each page as soon as it starts writing to it. For
MAP_SHARED mappings, all processes mapping the same file plus the kernel
page cache will write to the same page.

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Thread

Understanding the working of Shared memory using mmap() Pavankumar S V <pavankumarsv96@gmail.com> - 2023-03-09 04:19 -0800
  Re: Understanding the working of Shared memory using mmap() Richard Kettlewell <invalid@invalid.invalid> - 2023-03-10 16:30 +0000
  Re: Understanding the working of Shared memory using mmap() Rainer Weikusat <rweikusat@talktalk.net> - 2023-03-13 17:35 +0000

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