The following pull request was submitted through Github. It can be accessed and reviewed at: https://github.com/lxc/lxcfs/pull/436
This e-mail was sent by the LXC bot, direct replies will not reach the author unless they happen to be subscribed to this list. === Description (from pull-request) === Signed-off-by: Stéphane Graber <stgra...@ubuntu.com>
From 8630139a22a0cf580c4ffaf849db98ba51de5df2 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?St=C3=A9phane=20Graber?= <stgra...@ubuntu.com> Date: Wed, 4 Nov 2020 23:06:00 -0500 Subject: [PATCH] README: clarify SWAP MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Signed-off-by: Stéphane Graber <stgra...@ubuntu.com> --- README.md | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/README.md b/README.md index 5272a6c..11e93fd 100644 --- a/README.md +++ b/README.md @@ -104,3 +104,39 @@ docker run -it -m 256m --memory-swap 256m \ In a system with swap enabled, the parameter "-u" can be used to set all values in "meminfo" that refer to the swap to 0. sudo lxcfs -u /var/lib/lxcfs + +## Swap handling +Swap cgroup handling on Linux is very confusing and there just isn't a perfect way for LXCFS to handle it. + +The main issues are: + - SWAP accounting is opt-in and requires `swapaccount=1` be passed to the kernel. + + - Both a RAM limit and a RAM+SWAP limit can be set. The delta however + isn't the available SWAP space as the kernel is still free to SWAP as + much of the RAM as it feels like. This makes it impossible to render + a SWAP device size as using the delta between RAM and RAM+SWAP for that + wouldn't account for the kernel swapping more pages, leading to swap + usage exceeding swap total. + + - It's impossible to disable SWAP in a given container. The closest + that can be done is setting swappiness down to 0 which severly limits + the risk of swapping pages but doesn't eliminate it. + +As a result, LXCFS had to make some compromise which go as follow: + - When `swapaccount` isn't enabled, no SWAP space is reported at all. + This is simply because there is no way to know the SWAP consumption. + The container may very much be using some SWAP though, there's just + no way to know how much of it and showing a SWAP device would require + some kind of SWAP usage to be reported. Showing the host value would be + completely wrong, showing a 0 value would be equallty wrong. + + - Because SWAP usage for a given container can exceed the delta between + RAM and RAM+SWAP, the SWAP size is always reported to be the smaller of + the RAM limit or the host SWAP device itself. This ensures that at no point SWAP + usage will be allowed to exceed the SWAP size. + + - If the swappiness is set to 0 and there is no SWAP usage, no SWAP is reported. + However if there is SWAP usage, then a SWAP device of the size of the + usage (100% full) is reported. This provides adequate reporting of + the memory consumption while preventing applications from assuming more + SWAP is available.
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