Re: [PATCH v3 11/25] tools/xenstore: drop use of tdb

[Julien Grall <julien@xxxxxxx>]

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 <jgross@xxxxxxxx>
Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall