Hi Juergen,
On 24/07/2023 12:02, Juergen Gross wrote:
Today all Xenstore nodes are stored in a TDB data base. This data base
has several disadvantages:
- It is using a fixed sized hash table, resulting in high memory
overhead for small installations with only very few VMs, and a rather
large performance hit for systems with lots of VMs due to many
collisions.
The hash table size today is 7919 entries. This means that e.g. in
case of a simple desktop use case with 2 or 3 VMs probably far less
than 10% of the entries will be used (assuming roughly 100 nodes per
VM). OTOH a setup on a large server with 500 VMs would result in
heavy conflicts in the hash list with 5-10 nodes per hash table entry.
- TDB is using a single large memory area for storing the nodes. It
only ever increases this area and will never shrink it afterwards.
This will result in more memory usage than necessary after a peak of
Xenstore usage.
- Xenstore is only single-threaded, while TDB is designed to be fit
for multi-threaded use cases, resulting in much higher code
complexity than needed.
- Special use cases of Xenstore are not possible to implement with TDB
in an effective way, while an implementation of a data base tailored
for Xenstore could simplify some handling (e.g. transactions) a lot.
So drop using TDB and store the nodes directly in memory making them
easily accessible. Use a hash-based lookup mechanism for fast lookup
of nodes by their full path.
For now only replace TDB keeping the current access functions.
Signed-off-by: Juergen Gross <jgr...@suse.com>
Reviewed-by: Julien Grall <jgr...@amazon.com>
Cheers,
--
Julien Grall
