Re: [PATCH 0/3] Allow user to request memory to be locked on page fault

[Andrew Morton]

On Fri, 8 May 2015 16:06:10 -0400 Eric B Munson <emunson@xxxxxxxxxx> wrote:

> On Fri, 08 May 2015, Andrew Morton wrote:
> 
> > On Fri,  8 May 2015 15:33:43 -0400 Eric B Munson <emunson@xxxxxxxxxx> wrote:
> > 
> > > mlock() allows a user to control page out of program memory, but this
> > > comes at the cost of faulting in the entire mapping when it is
> > > allocated.  For large mappings where the entire area is not necessary
> > > this is not ideal.
> > > 
> > > This series introduces new flags for mmap() and mlockall() that allow a
> > > user to specify that the covered are should not be paged out, but only
> > > after the memory has been used the first time.
> > 
> > Please tell us much much more about the value of these changes: the use
> > cases, the behavioural improvements and performance results which the
> > patchset brings to those use cases, etc.
> > 
> 
> The primary use case is for mmaping large files read only.  The process
> knows that some of the data is necessary, but it is unlikely that the
> entire file will be needed.  The developer only wants to pay the cost to
> read the data in once.  Unfortunately developer must choose between
> allowing the kernel to page in the memory as needed and guaranteeing
> that the data will only be read from disk once.  The first option runs
> the risk of having the memory reclaimed if the system is under memory
> pressure, the second forces the memory usage and startup delay when
> faulting in the entire file.

Why can't the application mmap only those parts of the file which it
wants and mlock those?

> I am working on getting startup times with and without this change for
> an application, I will post them as soon as I have them.

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