Date: Thu, 3 Apr 2014 12:30:40 -0400 From: John Baldwin <jhb@freebsd.org> To: Ian Lepore <ian@freebsd.org> Cc: freebsd-hackers@freebsd.org, Dmitry Sivachenko <trtrmitya@gmail.com>, Trond =?iso-8859-1?q?Endrest=F8l?= <Trond.Endrestol@fagskolen.gjovik.no> Subject: Re: madvise() vs posix_fadvise() Message-ID: <201404031230.40380.jhb@freebsd.org> In-Reply-To: <1396539837.81853.278.camel@revolution.hippie.lan> References: <D6BD48AF-9522-495D-8D54-37854E53C272@gmail.com> <201404031102.38598.jhb@freebsd.org> <1396539837.81853.278.camel@revolution.hippie.lan>
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On Thursday, April 03, 2014 11:43:57 am Ian Lepore wrote: > On Thu, 2014-04-03 at 11:02 -0400, John Baldwin wrote: > > 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 > > Does that work well in the case of something like /var/log/messages that > is repeatedly appended-to at random intervals? It would be bad if every > new line written to the log triggered a physical read-modify-write. On > the other hand if it somehow results in the last / partitial block being > the only one likely to stay in memory, that would be perfect. The latter. It's sort of like a lazy O_DIRECT. Each time you call write(2), it tries to move any clean pages from your current sequentially written stream from inactive to cache, so the pages won't move until a subsequent write(2) after bufdaemon or the syncer actually forces them to be written. Unfortunately, it is currently implemented by doing an internal FADV_DONTNEED after each read() or write(). It would be better if it was implemented as a callback when buffers are completed. -- John Baldwin
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