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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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