Hello,
do we have objections to this still? I am fine with it.
Best, -Michael
Signed-off-by: Michael Tremer michael.tremer@ipfire.org
On Wed, 2018-09-26 at 20:55 +0100, Tim FitzGeorge wrote:
On 25/09/2018 21:50, Michael Tremer wrote:
Hello Tim,
welcome to the list and thanks for the patch.
On Tue, 2018-09-25 at 20:29 +0200, Peter Müller wrote:
Hello Tim,
I am not sure about the side-effects introduced with this patch.
As far as I am aware, values for "vm.swappiness" range from 0 (disable swapping entirely) to 100 (swap aggressively). Value 1 means swapping is only used to avoid OOM situations.
No, 0 does not disable swapping entirely.
https://git.ipfire.org/?p=thirdparty/kernel/linux.git;a=blob;f=Documentation...
Yes, with swappiness=0 the system will start swapping when the amount of free memory drops below a calculated value, but it will not handle a demand for more than the available amount of free memory. In this case it calls the OOM killer.
With swappiness=1 it will start swapping memory out to make room rather than OOM.
With swappiness>=2 the system will start pre-emptively swapping out memory pages that it doesn't think are needed any more. In principle this makes for a faster system because it doesn't have to stop to swap out memory when it needs to find some, but obviously there's a problem if it's swapped out something that's actually needed.
It's worth noting that while the kernel doesn't load pages of an executable until they're actually used, it has to load any initialisation code. This initialisation code is left in memory when the program is running. There is therefore an argument that _under nominal conditions_ a slightly larger value of swappiness (for example 10) would give the best results, since it would allow initialisation and other code to be swapped out, while still keeping memory that's being used.
Of course, as Peter has said we also need to consider non-nominal conditions.
This raises three questions: (a) On some systems, I observer swap usage > 0% indeed, which should not happen if value 0 for this setting _disables_ swapping.
See above.
(b) Since disk I/O is much slower than RAM access, I fear it might be a DoS vector to enable this (infected program running amok). On the other hand, if a systems might avoid running out of memory, this sounds good too.
Not swapping would cause the OOM killer to kill random processes. That also is a DoS attack vector. Every OOM situation is. There is no way to recover from this.
I think it's really a trade-off. With swappiness=0 a random process is killed, which could well reduce the amount of protection that IPFire is giving you. With swappiness=1 there's a longer pause while swapping is carried out (the OOM killer also takes time), but the amount of protection stays the same. Note that with swappiness=0 we could also set oom_kill_allocating_task to kill the task that triggers the event;this would be quicker but still potentially leads to a loss of protection.
Which is preferable probably depends on what you're protecting. If you're protecting the IPFire source, the nightmare scenario is someone installing a backdoor in the code under the cover of a DOS attack - so you'd likely prefer swappiness=1. If you're protecting a home network that's mainly used for gaming, you may well decide that increased risk with swappiness=0 is acceptable.
Obviously, this only applies to a one off instance of swapping if your system is swapping continually, then you need more memory as Michael says below.
(c) How does the kernel treat anonymous pages after changing this setting?
Anonymous pages?
The system can swap out pages of executable code - this just requires marking the page as unused since if it's needed again it can just be reloaded from the file on disk, or data. Data pages are known as anonymous whereas program pages can be considered to have the name of the executable.
I don't think that setting swappiness=1 affects this. The kernel will try to swap out pages that it thinks are least likely to be needed; if these are anonymous pages they get written to the swap file, otherwise they're just freed.
However, these might be academic threats since the overall issue is already discussed in https://bugzilla.ipfire.org/show_bug.cgi?id=11839 . Just some comments from my side... :-)
Not really. I guess what we really want is 1 here. There is usually no reason for the firewall to swap. That only happens for the proxy or IDS (or an application with a massive memory leak). Usually that can be configured away or the amount of RAM in the machine has to be upgraded.
As long as there is enough memory available, we want to keep everything in memory. There is no point in swapping out the IDS ruleset or proxy cache in memory. We only will do this to keep the system alive if there is a peak in memory usage and that is what vm.swappiness=1 is doing from my POV.
Best, -Michael
In my scenario I've got a script that downloads IDS rules, assesses and the applies the changes, runs snort -T to check that the updated rule files are OK and then tells the running instances of Snort to re-read the rules. Its peak memory usage (during snort -T) is around 500MB. I'm happy for the system to swap when it's doing this, even if it does slow things down (mind you, the downloading of a almost 100MB set of rules will slow traffic over the red interface anyway).
With swappiness=0 I would get the OOM killer triggered (it would usually kill one of the snort instances). Since setting swappiness=1 this doesn't happen any more, and my swap partition usage has stayed at 0. From my point of view, everything is behaving correctly.
It's a pity that the WUI doesn't include a graph of swap rate alongside the swap usage graph - it would be useful for this discussion.
Best regards,
Tim
Thanks, and best regards, Peter Müller
Signed-off-by: Tim FitzGeorge <ipfr at tfitzgeorge.me.uk> Fixes: Bug 11839
config/etc/sysctl.conf | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/config/etc/sysctl.conf b/config/etc/sysctl.conf index 345f8f52a..4066af767 100644 --- a/config/etc/sysctl.conf +++ b/config/etc/sysctl.conf @@ -27,7 +27,7 @@ net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.all.log_martians = 1
kernel.printk = 1 4 1 7 -vm.swappiness=0 +vm.swappiness=1 vm.mmap_min_addr = 4096 vm.min_free_kbytes = 8192