>
> Hi Honnappa,
>
> Reply inlined:
Hi Yipeng,
Thank you so much for reviewing.
>
> >-----Original Message-----
> >
> > Currently, reader-writer concurrency problems in rte_hash are
> > addressed using reader-writer locks. Use of reader-writer locks
> > results in following issues:
> >
> > 1) In many of the use cases for the hash table, writer threads
> > are running on control plane. If the writer is preempted while
> > holding the lock, it will block the readers for an extended period
> > resulting in packet drops. This problem seems to apply for platforms
> > with transactional memory support as well because of the algorithm
> > used for rte_rwlock_write_lock_tm:
> >
> > static inline void
> > rte_rwlock_write_lock_tm(rte_rwlock_t *rwl)
> > {
> > if (likely(rte_try_tm(&rwl->cnt)))
> > return;
> > rte_rwlock_write_lock(rwl);
> > }
> >
> > i.e. there is a posibility of using rte_rwlock_write_lock in
> > failure cases.
> [Wang, Yipeng] In our test, TSX failure happens very rarely on a TSX
> platform. But we agree that without TSX, the current rte_rwlock
> implementation may make the writer to hold a lock for a period of time.
>
> > 2) Reader-writer lock based solution does not address the following
> > issue.
> > rte_hash_lookup_xxx APIs return the index of the element in
> > the key store. Application(reader) can use that index to reference
> > other data structures in its scope. Because of this, the
> > index should not be freed till the application completes
> > using the index.
> [Wang, Yipeng] I agree on this use case. But I think we should provide new
> API functions for deletion to users who want this behavior, without
> changing the meaning of current API if that is possible.
In the lock-free algorithm, the rte_hash_delete API will not free the index.
The new API rte_hash_free will free the index. The solution for the algorithm
with rw locks needs to be thought about.
>
> > Current code:
> > Cores Lookup Lookup
> > with add
> > 2 474 246
> > 4 935 579
> > 6 1387 1048
> > 8 1766 1480
> > 10 2119 1951
> > 12 2546 2441
> >
> > With this patch:
> > Cores Lookup Lookup
> > with add
> > 2 291 211
> > 4 297 196
> > 6 304 198
> > 8 309 202
> > 10 315 205
> > 12 319 209
> >
> [Wang, Yipeng] It would be good if you could provide the platform
> information on these results.
Apologies, I should have done that. The machine I am using is: Intel(R) Xeon(R)
CPU E5-2660 v4 @ 2.00GHz, 64G memory. This is a hacked test case which is not
upstreamed. In the case of 'Lookup with add' - I had equal number of threads
calling 'rte_hash_add' and 'rte_hash_lookup'. In the case of 'Lookup' - a set
of entries were added and all the threads called 'rte_hash_lookup'. Note that
these are the numbers without htm. We have created another test case which I
will upstream as next version of this patch. I will publish the numbers with
that test case. So, you should be able to reproduce the numbers with that test
case.
>
> Another comment is as you know we also have a new extension to rte_hash
> to enable extendable buckets and partial-key hashing. Thanks for the
> comments btw. Could you check if your lockless scheme also applies to
> those extensions?
Thank you for reminding me on this. I thought I had covered everything. On a
relook, I have missed few key issues. I will reply on the other email thread.
