[pve-devel] [PATCH] ZFS ARC size not taken into account by pvestatd or ksmtuned

[Stephane Chazelas]

[note that I'm not subscribed to the list]

Hello,

I've been meaning to send this years ago. Sorry for the delay.

We've been maintaining the patch below on our servers for years
now (since 2015), even before ZFS was officially supported by
PVE.

We had experienced VM balloons swelling and processes in VMs
running out of memory even though the host had tons of RAM.

We had tracked that down to pvestatd reclaiming memory from the
VMs. pvestatd targets 80% memory utilisation (in terms of memory
that is not free and not in buffers or caches: (memtotal -
memfree - buffers - cached) / memtotal).

The problem is that the ZFS ARC is tracked independendly (not as
part of "buffers" or "cached" above).

The size of that ARC cache also adapts with memory pressure. But
here, since the autoballooning frees memory as soon as it's used
up by the ARC, the ARC size grows and grows while VMs access
their disk, and we've got plenty of wasted free memory that is
never used.

So in the end, with an ARC allowed to grow up to half the RAM,
we end up in a situation where pvestatd in effect targets 30%
max memory utilisation (with 20% free or in buffers and 50% in
ARC).

Something similar happens for KSM (memory page deduplication).
/usr/sbin/ksmtuned monitors memory utilisation (again
total-cached-buffers-free) against kvm process memory
allocation, and tells the ksm daemon to scan more and more
pages, more and more aggressively as long as the "used" memory
is above 80%.

That probably explains why performances decrease significantly
after a while and why doing a "echo 3 >
/proc/sys/vm/drop_caches" (which clears buffers, caches *AND*
the ZFS arc cache) gives a second life to the system.

(by the way, a recent version of ProcFSTools.pm added a
read_pressure function, but it doesn't look like it's used
anywhere).

--- /usr/share/perl5/PVE/ProcFSTools.pm.distrib	2020-12-03 15:53:17.000000000 +0000
+++ /usr/share/perl5/PVE/ProcFSTools.pm	2021-01-19 13:44:42.480272044 +0000
@@ -268,6 +268,19 @@ sub read_meminfo {
 
     $res->{memtotal} = $d->{memtotal};
     $res->{memfree} =  $d->{memfree} + $d->{buffers} + $d->{cached};
+
+    # Add the ZFS ARC if any
+    if (my $fh_arc = IO::File->new("/proc/spl/kstat/zfs/arcstats", "r")) {
+	while (my $line = <$fh_arc>) {
+	    if ($line =~ m/^size .* (\d+)/) {
+	        # "size" already in bytes
+		$res->{memfree} += $1;
+		last;
+	    }
+	}
+	close($fh_arc);
+    }
+
     $res->{memused} = $res->{memtotal} - $res->{memfree};
 
     $res->{swaptotal} = $d->{swaptotal};
--- /usr/sbin/ksmtuned.distrib	2020-07-24 10:04:45.827828719 +0100
+++ /usr/sbin/ksmtuned	2021-01-19 14:37:43.416360037 +0000
@@ -75,10 +75,17 @@ committed_memory () {
     ps -C "$progname" -o vsz= | awk '{ sum += $1 }; END { print sum }'
 }
 
-free_memory () {
-    awk '/^(MemFree|Buffers|Cached):/ {free += $2}; END {print free}' \
-                /proc/meminfo
-}
+free_memory () (
+    shopt -s nullglob
+    exec awk '
+      NR == FNR {
+          if (/^(MemFree|Buffers|Cached):/) free += $2
+          next
+      }
+      $1 == "size" {free += int($3/1024)}
+      END {print free}
+      ' /proc/meminfo /proc/spl/kstat/zfs/[a]rcstats
+)
 
 increase_npages() {
     local delta