Date: Thu, 3 Apr 2014 11:02:38 -0400 From: John Baldwin <jhb@freebsd.org> To: Dmitry Sivachenko <trtrmitya@gmail.com> Cc: freebsd-hackers@freebsd.org, Trond =?utf-8?q?Endrest=C3=B8l?= <Trond.Endrestol@fagskolen.gjovik.no> Subject: Re: madvise() vs posix_fadvise() Message-ID: <201404031102.38598.jhb@freebsd.org> In-Reply-To: <0AF273E6-CD43-417C-A00C-5B7445090D5B@gmail.com> References: <D6BD48AF-9522-495D-8D54-37854E53C272@gmail.com> <201403271141.41487.jhb@freebsd.org> <0AF273E6-CD43-417C-A00C-5B7445090D5B@gmail.com>
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On Thursday, April 03, 2014 7:29:03 am Dmitry Sivachenko wrote: > > On 27 марта 2014 г., at 19:41, John Baldwin <jhb@FreeBSD.org> wrote: > >> > >> I know about mlock(2), it is a bit overkill. > >> Can someone please explain the difference between madvise(MADV_WILLNEED) and > > posix_fadvise(POSIX_FADV_WILLNEED)? > > > > Right now FADV_WILLNEED is a nop. (I have some patches to implement it for > > UFS.) I can't recall off the top of my head if MADV_WILLNEED is also a nop. > > However, if both are fully implemented they should be similar in terms of > > requesting async read-ahead. MADV_WILLNEED might also conceivably > > pre-create PTEs while FADV_WILLNEED can be used on a file that isn't > > mapped but is accessed via read(2). > > > > > Hello and thanks for your reply. > > 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. -- John Baldwin
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