Has anyone seen performance data comparing rte_hash (perhaps with r/w
locks) versus URCU hash?
On Mon, Apr 23, 2018 at 4:50 PM, Stephen Hemminger <
step...@networkplumber.org> wrote:
> On Mon, 23 Apr 2018 12:40:41 -0700
> Brijesh Singh <brijesh.s.si...@gmail.com> wrote:
>
> > A gentle reminder,
> >
> > I am curious to know if/how rte_hash is thread safe for lookups.It is
> > not obvious to me how following code is thread safe:
> >
> > _rte_hash_lookup_with_hash(const struct rte_hash *h, const void *key,
> >
> > hash_sig_t sig, void **data)
> >
> > {
> >
> >
> >
> > …
> >
> > if (rte_hash_cmp_eq(key, k->key, h) == 0) {
> >
> > if (data != NULL)
> >
> > *data = k->pdata;
> >
> > }
> >
> > a key could be deleted and another key inserted in its slot while the
> > lookup is happening. For example, in the following sequence of events:
> > The slot has Key1,V1
> > Lookup Thread T1 compares the input key to Key1 and it matches. The
> > thread gets context switched out
> > Thread T2 deletes Key1.
> > Thread T2 inserts Key2 with value V2.
> > T1 reads the data from the slot and returns V2. This is incorrect.
> >
> >
> > Regards,
> > Brijesh
> >
> > On Wed, Apr 11, 2018 at 9:12 PM, Brijesh Singh
> > <brijesh.s.si...@gmail.com> wrote:
> > > Hello,
> > >
> > > I want to use DPDK's rte_hash library to keep track of tcp flows. The
> > > lookups will be done by multiple threads but inserts will be done only
> > > on one thread.
> > >
> > > As per the documentation rte_hash library has thread safe lookups. Key
> > > /data inserts should be done on single thread, since those operations
> > > are not thread safe. Is this documentation still correct?
> > >
> > > The lookup code compares the key and returns the data if the key
> > > matches, this doesn't look like thread safe. Am I missing something?
> > >
> > > _rte_hash_lookup_with_hash(const struct rte_hash *h, const void *key,
> > >
> > > hash_sig_t sig, void **data)
> > >
> > > {
> > >
> > >
> > >
> > > …
> > >
> > > if (rte_hash_cmp_eq(key, k->key, h) == 0) {
> > >
> > > if (data != NULL)
> > >
> > > *data = k->pdata;
> > >
> > > }
> > >
> > > Regards,
> > > Brijesh
>
> The best way to handle this is to do some kind of Read Copy Update.
> Unfortunately, this is not possible in scope of DPDK since it requires
> cooperation from application threads.
>
> If you need thread safe hash table, my recommendation would be to
> skip the DPDK hash library and use userspace RCU instead:
> http://liburcu.org/
>
> Note: URCU is LGPL versus BSD licensed. But then any non trivial
> Linux application needs to use LGPL libraries already.
>
