Date: Fri, 4 Apr 2014 21:52:09 +0400 From: Dmitry Sivachenko <trtrmitya@gmail.com> To: John Baldwin <jhb@FreeBSD.org> Cc: freebsd-hackers@freebsd.org, =?utf-8?Q?Trond_Endrest=C3=B8l?= <Trond.Endrestol@fagskolen.gjovik.no> Subject: Re: madvise() vs posix_fadvise() Message-ID: <EF134BCA-1E92-4C98-8763-9A31EA96839A@gmail.com> In-Reply-To: <201404031102.38598.jhb@freebsd.org> References: <D6BD48AF-9522-495D-8D54-37854E53C272@gmail.com> <201403271141.41487.jhb@freebsd.org> <0AF273E6-CD43-417C-A00C-5B7445090D5B@gmail.com> <201404031102.38598.jhb@freebsd.org>
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On 03 =D0=B0=D0=BF=D1=80. 2014 =D0=B3., at 19:02, John Baldwin = <jhb@FreeBSD.org> wrote: >>=20 >> Right now I am facing the following problem (stable/10): >> There is a (home-grown) webserver which mmap's a large amount of data = files (total size is a bit below of RAM, say ~90GB of files with 128GB = of RAM). >> Server writes access.log (several gigabytes per day). >>=20 >> Some of mmaped data files are used frequently, some are used rarely. = On startup, server walks through all of these data files so it's content = is read=20 > from disk. >>=20 >> After some time of running, I see that rarely used data files are = purged from RAM (access to them leads to long-running disk reads) in = favour of disk=20 > cache >> (at 0:00, when I rotate and gzip log file I see Inactive memory goes = down to the value of log file size). >>=20 >> Is there any way to tell VM system not to push mmap'ed regions out of = RAM in favour of disk caches? >=20 > Use POSIX_FADV_NOREUSE with fadvise() for the log files. They are a = perfect > use case for this flag. This will tell the VM system to throw the log = data > (move it to cache) after it writes the file. Another question is why madvise(MADV_WILLNEED) is not enough to prefer = keeping mmap'ed data in memory instead of dedicating all memory to cache = log files? Even if that mmap'ed memory is rarely used. While POSIX_FADV_NOREUSE might be a solution for some cases (I am = already testing it), it needs to be implemented in many programs (all = that read/write files on disk), while madvise(MADV_WILLNEED) sounds like a proper solution to increase = priority for mmaped region regardless of what other programs use disk = but it does not seem to work as expected.=
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