[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Xen-devel] [PATCH] docs: Mention a common pitfall in ballooning



Several users have reported that their available free memory in the
guest when they used maxmem >> memory was much smaller than when
maxmem == memory.  This is the unavoidable consequence of how
ballooning works, but it's not something users expect.

Warn them of this effect in the place we tell them how to make it
happen, so they aren't surprised.

Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
---
CC: Ian Campbell <ian.campbell@xxxxxxxxxx>
CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 docs/man/xl.cfg.pod.5 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
index 93cd7d2..4432f95 100644
--- a/docs/man/xl.cfg.pod.5
+++ b/docs/man/xl.cfg.pod.5
@@ -264,6 +264,13 @@ if the values of B<memory=> and B<maxmem=> differ.
 A "pre-ballooned" HVM guest needs a balloon driver, without a balloon driver
 it will crash.
 
+NOTE: Because of the way ballooning works, the guest has to allocate
+memory to keep track of maxmem pages, regardless of how much memory it
+actually has available to it.  A guest with maxmem=262144 and
+memory=8096 will report significantly less memory available for use
+than a system with maxmem=8096 memory=8096 due to the memory overhead
+of having to track the unused pages.
+
 =back
 
 =head3 Guest Virtual NUMA Configuration
-- 
1.9.1


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.