Date: Sun, 21 Aug 2016 11:28:42 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Karl Denninger <karl@denninger.net> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS ARC under memory pressure Message-ID: <20160821082842.GR83214@kib.kiev.ua> In-Reply-To: <97f166f0-4d47-d5a3-ecb3-d15f1ecf9c1f@denninger.net> References: <20160816193416.GM8192@zxy.spb.ru> <8dbf2a3a-da64-f7f8-5463-bfa23462446e@FreeBSD.org> <20160818202657.GS8192@zxy.spb.ru> <c3bc6c5a-961c-e3a4-2302-f0f7417bc34f@denninger.net> <20160819201840.GA12519@zxy.spb.ru> <bcb14d0b-bd6d-cb93-ea71-3656cfce8b3b@denninger.net> <20160820152225.GP83214@kib.kiev.ua> <97f166f0-4d47-d5a3-ecb3-d15f1ecf9c1f@denninger.net>
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On Sat, Aug 20, 2016 at 11:08:44AM -0500, Karl Denninger wrote: > If you are talking about the case of an executable in which part of the > text is evicted you are correct, however, you are still choosing in that > instance to evict a page for which there will likely be a future demand > and thus require an I/O (should that executable come back up for > execution) as opposed to one for which you have no idea how likely > demand for same will be (a data page in the ARC.) No, I am not talking about only text segments. Any clean page can be reused after unmapping. > > Since the VM has no means of "coloring" the ARC (as it is opaque other > than the consumption of system memory to the VM) as to how "useful" > (e.g. how often used, etc) a particular data item in the ARC is, it has > no information available on which to decide. However, the fact that an > executing process is in some sort of waiting state still likely trumps > an ARC data page in terms of likelihood of future access. Buffer cache behaves exactly the same, since access references are not counted for the pages constituing buffers.
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