Strange memory management with mmap()

[Dmitry Sivachenko]

> On 16 июля 2015 г., at 21:19, Dmitry Sivachenko <trtrmitya at gmail.com> wrote:
> 
>> 
>> 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.
>> 
> 
> 


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.