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List:       freebsd-hackers
Subject:    Re: Strange memory management with mmap()
From:       Dmitry Sivachenko <trtrmitya () gmail ! com>
Date:       2015-07-22 12:47:12
Message-ID: 15DE3B94-3C09-4855-A274-D5655B049403 () gmail ! com
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> On 16 июля 2015 г., at 21:19, Dmitry Sivachenko <trtrmitya@gmail.com> wrote:
> 
> > 
> > On 16 июля 2015 г., at 18:42, Dmitry Sivachenko <trtrmitya@gmail.com> wrote:
> > 
> > Hello!
> > 
> > I am using FreeBSD-10-stable and writing a program that uses large data file via \
> > mmap() in read only mode. To be specific, I have 256GB RAM machine and typical \
> > size of data file is ~160GB (more than 1/2 of RAM and less that the whole RAM). \
> > There is no other programs running during the test. 
> > Consider the following use case: I have two files on disk.  I mmap() the first \
> > one and prefetch data to RAM (touch every page of the file). After that I expect \
> > all data to be cached in RAM and subsequent access will be fast. 
> > Next I do munmap() on the first file, mmap() the second one and do the same test: \
> > prefetch data and expect it to be cached in RAM (and some of the pages belonging \
> > to the first file to be purged out, because size_of(file1)+size_of(file2) > \
> > size_of(RAM). 
> > Please find my test program attached.
> > 
> > I run the program with 2 files provided via command line (both about 160GB).
> > What I observe in real is:
> > -- before I run the program all RAM is in FREE state as reported by top(1).
> > -- after first prefetch() of the first file, all it's data goes to "Cache" state, \
> >                 RES column of the process remains the same (small)
> > -- second prefetch() works fast as expected, memory goes from Cache to Active \
> >                 state, RES column of the process grows up to match file size \
> >                 (SIZE==RES now)
> > -- now first prefetch() for second file starts: the remaining Free memory goes to \
> >                 Cache state, Active size still equals to first file size.
> > -- second prefetch() for second file works as slow as first one, like if nothing \
> > was cached in memory during the first prefetch() run, RES column does not change. \
> >  
> > Here is the output:
> > % /tmp/a.out file1.dat file2.dat
> > file1.dat... First prefault time: 1235.747351 seconds
> > Second prefault time: 74.893323 seconds
> > Ok.
> > file2.dat... First prefault time: 1316.405527 seconds
> > Second prefault time: 1311.491842 seconds
> > Ok.
> > 
> 
> 


I tried the same test program on Linux machine with similar hardware.  It behaves \
like expected (second prefetch works very fast):

file1.dat... First prefault time: 2664.621088 seconds
Second prefault time: 1.969283 seconds
Ok.
file2.dat... First prefault time: 2917.009003 seconds
Second prefault time: 34.128762 seconds
Ok.


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