Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU

From: Arnd Bergmann <arnd@arndb.de>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Nishanth Menon <nm@ti.com>,
	Santosh Shilimkar <santosh.shilimkar@oracle.com>,
	Tero Kristo <t-kristo@ti.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Michal Hocko <mhocko@suse.com>, Rik van Riel <riel@surriel.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Santosh Shilimkar <ssantosh@kernel.org>,
	Dave Chinner <david@fromorbit.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>, Yafang Shao <laoar.shao@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	kernel-team@fb.com, Kishon Vijay Abraham I <kishon@ti.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Roman Gushchin <guro@fb.com>
Subject: Re: [PATCH] vfs: keep inodes with page cache off the inode shrinker LRU
Date: Mon, 9 Mar 2020 14:33:26 +0100
Message-ID: <CAK8P3a2Gz5H_fcNtW0yCCjO1cRNa0nyd568sDYR0nNphu49YqQ@mail.gmail.com> (raw)
In-Reply-To: <20200308141923.GI25745@shell.armlinux.org.uk>

On Sun, Mar 8, 2020 at 3:20 PM Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
> On Sun, Mar 08, 2020 at 11:58:52AM +0100, Arnd Bergmann wrote:
> > On Fri, Mar 6, 2020 at 9:36 PM Nishanth Menon <nm@ti.com> wrote:
> > > On 13:11-20200226, santosh.shilimkar@oracle.com wrote:
>
> > - extend zswap to use all the available high memory for swap space
> >   when highmem is disabled.
>
> I don't think that's a good idea.  Running debian stable kernels on my
> 8GB laptop, I have problems when leaving firefox running long before
> even half the 16GB of swap gets consumed - the entire machine slows
> down very quickly when it starts swapping more than about 2 or so GB.
> It seems either the kernel has become quite bad at selecting pages to
> evict.
>
> It gets to the point where any git operation has a battle to fight
> for RAM, despite not touching anything else other than git.
>
> The behaviour is much like firefox is locking memory into core, but
> that doesn't seem to be what's actually going on.  I've never really
> got to the bottom of it though.
>
> This is with 64-bit kernel and userspace.

I agree there is something going wrong on your machine, but I
don't really see how that relates to my suggestion.

> So, I'd suggest that trading off RAM available through highmem for VM
> space available through zswap is likely a bad idea if you have a
> workload that requires 4GB of RAM on a 32-bit machine.

Aside from every workload being different, I was thinking of
these general observations:

- If we are looking at a future without highmem, then it's better to use
  the extra memory for something than not using it. zswap seems like
  a reasonable use.

- A lot of embedded systems are configured to have no swap at all,
  which can be for good or not-so-good reasons. Having some
  swap space available often improves things, even if it comes
  out of RAM.

- A particularly important case to optimize for is 2GB of RAM with
  LPAE enabled. With CONFIG_VMSPLIT_2G and highmem, this
  leads to the paradox -ENOMEM when 256MB of highmem are
  full while plenty of lowmem is available. With highmem disabled,
  you avoid that at the cost of losing 12% of RAM.

- With 4GB+ of RAM and CONFIG_VMSPLIT_2G or
  CONFIG_VMSPLIT_3G, using gigabytes of RAM for swap
  space would usually be worse than highmem, but once
  we have VMSPLIT_4G_4G, it's the same situation as above
  with 6% of RAM used for zswap instead of highmem.

       Arnd