Date: Thu, 16 Jul 2015 21:19:13 +0300 From: Dmitry Sivachenko <trtrmitya@gmail.com> To: hackers@freebsd.org Subject: Re: Strange memory management with mmap() Message-ID: <3434ED75-7994-4E9E-9B06-FACCD7DC90FF@gmail.com> In-Reply-To: <FDB6E0F9-A3FF-4194-83C1-A3121CBAE407@gmail.com> References: <FDB6E0F9-A3FF-4194-83C1-A3121CBAE407@gmail.com>
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> On 16 =D0=B8=D1=8E=D0=BB=D1=8F 2015 =D0=B3., at 18:42, Dmitry = Sivachenko <trtrmitya@gmail.com> wrote: >=20 > Hello! >=20 > 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. >=20 > 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. >=20 > 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). >=20 > Please find my test program attached. >=20 > 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=3D=3DRES 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. >=20 >=20 > 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. >=20 And one more strange observation I made:=20 if I add=20 if (madvise(p, st.st_size, MADV_RANDOM) !=3D 0) err(1, "madvise"); if (madvise(p, st.st_size, MADV_WILLNEED) !=3D 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.
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