Re: [PATCH 10/11] tools/xenstore: drop use of tdb

[Juergen Gross <jgross@xxxxxxxx>]

On 29.06.23 13:30, Juergen Gross wrote:
> On 20.06.23 15:09, Julien Grall wrote:
> > Hi Juergen,
> >
> > On 30/05/2023 10:13, 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
> > >
> > > - 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>
...

> > > -    set_tdb_key(db_name, &key);
> > > -    data = tdb_fetch(tdb_ctx, key);
> > > -    if (!data.dptr)
> > > -        errno = (tdb_error(tdb_ctx) == TDB_ERR_NOEXIST) ? ENOENT : EIO;
> > > -    else
> > > -        *size = data.dsize;
> > > +    hdr = hashtable_search(nodes, db_name);
> > > +    if (!hdr) {
> > > +        errno = ENOENT;
> > > +        return NULL;
> > > +    }
> > > +
> > > +    *size = sizeof(*hdr) + hdr->num_perms * sizeof(hdr->perms[0]) +
> > > +        hdr->datalen + hdr->childlen;
> > AFAICT, this is the same computation as in write_node_raw. Can we introduce a 
> > wrapper for it?
> Okay.
There is a difference: here we are using the values from xs_tdb_record_hdr,
while in write_node_raw() the values are taken from struct node.

I'll change that in a followup patch.


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature