[libvirt] [PATCH] Increase default file handle limits for daemons

[Daniel P. Berrange]

Linux still defaults to a 1024 open file handle limit. This causes
scalability problems for libvirtd / virtlockd / virtlogd on large
hosts which might want > 1024 guest to be running. In fact if each
guest needs > 1 FD, we can't even get to 500 guests. This is not
good enough when we see machines with 100's of physical cores and
TBs of RAM.

In comparison to other memory requirements of libvirtd & related
daemons, the resource usage associated with open file handles
is essentially line noise. It is thus reasonable to increase the
limits unconditionally for all installs.

Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
---
 daemon/libvirtd.service.in       | 7 +++++--
 src/locking/virtlockd.service.in | 4 ++++
 src/logging/virtlogd.service.in  | 5 +++++
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/daemon/libvirtd.service.in b/daemon/libvirtd.service.in
index c72dde5..22fc156 100644
--- a/daemon/libvirtd.service.in
+++ b/daemon/libvirtd.service.in
@@ -24,8 +24,11 @@ ExecStart=@sbindir@/libvirtd $LIBVIRTD_ARGS
 ExecReload=/bin/kill -HUP $MAINPID
 KillMode=process
 Restart=on-failure
-# Override the maximum number of opened files
-#LimitNOFILE=2048
+# At least 1 FD per guest, often 2 (eg qemu monitor + qemu agent).
+# If we want to support 2048 guests, we'll typically need 4096 FDs
+# If changing this, also consider virtlogd.service & virtlockd.service
+# limits which are also related to number of guests
+LimitNOFILE=8192
 
 [Install]
 WantedBy=multi-user.target
diff --git a/src/locking/virtlockd.service.in b/src/locking/virtlockd.service.in
index 69b568f..c369591 100644
--- a/src/locking/virtlockd.service.in
+++ b/src/locking/virtlockd.service.in
@@ -13,6 +13,10 @@ ExecReload=/bin/kill -USR1 $MAINPID
 # cause the machine to be fenced (rebooted), so make
 # sure we discourage OOM killer
 OOMScoreAdjust=-900
+# Needs to allow for max guests * average disks per guest
+# libvirtd.service written to expect 4096 guests, so if we
+# allow for 4 disks per guest, we get:
+LimitNOFILE=16384
 
 [Install]
 Also=virtlockd.socket
diff --git a/src/logging/virtlogd.service.in b/src/logging/virtlogd.service.in
index 09e0740..be039c6 100644
--- a/src/logging/virtlogd.service.in
+++ b/src/logging/virtlogd.service.in
@@ -13,6 +13,11 @@ ExecReload=/bin/kill -USR1 $MAINPID
 # cause the machine to be fenced (rebooted), so make
 # sure we discourage OOM killer
 OOMScoreAdjust=-900
+# Need to have at least one file open per guest (eg QEMU
+# stdio log), but might be more (eg serial console logs)
+# libvirtd.service written to expect 2048 guests, so if we
+# guess at 2 log files per guest here (stdio + 1 serial):
+LimitNOFILE=8192
 
 [Install]
 Also=virtlogd.socket
-- 
2.9.3