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 апр. 2014 г., at 19:02, John Baldwin <jhb@FreeBSD.org> wrote: >> >> 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). >> >> 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 > from disk. >> >> 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 > cache >> (at 0:00, when I rotate and gzip log file I see Inactive memory goes down to the value of log file size). >> >> Is there any way to tell VM system not to push mmap'ed regions out of RAM in favour of disk caches? > > 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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