Strange memory management with mmap()

[Dmitry Sivachenko]

> On 16 июля 2015 г., at 18:42, Dmitry Sivachenko <trtrmitya at 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.
> 


And one more strange observation I made: 
if I add 

if (madvise(p, st.st_size, MADV_RANDOM) != 0)
                err(1, "madvise");
if (madvise(p, st.st_size, MADV_WILLNEED) != 0)
                err(1, "madvise");

after mmap() call in my test program, these times change as follows:

% /tmp/a.out file1.dat file2.dat
file1.dat... First prefault time: 2706.270454 seconds
Second prefault time: 3.812012 seconds
Ok.
file2.dat... First prefault time: 4089.096647 seconds
Second prefault time: 1132.210888 seconds
Ok.