Re: [dpdk-dev] [PATCH 4/4] hash: enable lock-free reader-writer concurrency

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> >> >
> >> >Add the flag to enable reader-writer concurrency during run time.
> >> >The rte_hash_del_xxx APIs do not free the keystore element when this
> >> >flag is enabled. Hence a new API, rte_hash_free_key_with_position,
> >> >to free the key store element is added.
> >> >
> >> >+/** Flag to support lock free reader writer concurrency */ #define
> >> >+RTE_HASH_EXTRA_FLAGS_RW_CONCURRENCY_LF 0x08
> >> [Wang, Yipeng] It would be good to indicate that the lockless
> >> implementation works for single writer multiple readers.
> >Multi-writers are supported by using the rw-lock or transactional
> >memory. Essentially, we still have single writer. This patch works fine with
> multi-writer as defined by ' MULTI_WRITER_ADD' flag. I have tested it as well.
> I will enable this test case in V2.
> >
> >> Also, if people use a mix of the flags for example set both
> >> multiwriter and LF flags, then I guess either we need to return an
> >> error or maybe multiwriter should have higher priority. Currently the
> >> RW_CONCURRENCY will assume MULTI_WRITER_ADD I think.
> >As mentioned above, multi-writer and LF combination is supported. Yes,
> RW_CONCURRENCY currently assumes MULTI_WRITER_ADD.
> >I think we should separate them.
> [Wang, Yipeng] It would be great if you could just add a little bit more
> comments to both of the flags to be more specific on what Read write
> concurrency mean in both cases, just in case users got confused.
> You may also want to update the documentation later
> (https://doc.dpdk.org/guides/prog_guide/hash_lib.html).
I will add the documentation once the patch is accepted.

> 
> >
> >> >+
> >> > /** Signature of key that is stored internally. */ typedef uint32_t
> >> > hash_sig_t;
> >> >
> >> >@@ -143,6 +148,11 @@ rte_hash_count(const struct rte_hash *h);
> >> >  * and should only be called from one thread by default.
> >> >  * Thread safety can be enabled by setting flag during
> >> >  * table creation.
> >> >+ * When lock free reader writer concurrency is enabled,
> >> >+ * if this API is called to update an existing entry,
> >> >+ * the application should free any memory allocated for
> >> >+ * previous 'data' only after all the readers have stopped
> >> >+ * using previous 'data'.
> >> [Wang, Yipeng] Could you be more specific on this description?
> >> When add_key API is called, the users do not know if it will update
> >> an existing entry or inserting a new one, do they?
> >I think, it will depend on the application. The applications I have
> >worked on so far, added a hash entry as a result of receiving an event
> >and updated it on receiving another event. I can change the comments to
> indicate that the applications need to be aware of add/update operations.
> [Wang, Yipeng] Even if for current rte_hash, after update, the application may
> still use the old data. It is the upper level application's Responsibility. How is it
> specific to lock free implementation?
I agree. I think it makes sense to keep this warning, but make it not specific to lock-free algorithm. I will make this change in V3.

> >
> >> > rte_hash_del_key(const struct rte_hash *h, const void *key); @@
> >> > -251,6
> >> >+274,12 @@ rte_hash_del_key(const struct rte_hash *h, const void
> >> >+*key);
> >> >  * and should only be called from one thread by default.
> >> >  * Thread safety can be enabled by setting flag during
> >> >  * table creation.
> >> >+ * If lock free reader writer concurrency is enabled,
> >> >+ * the hash library's internal memory for the deleted
> >> >+ * key is not freed. It should be freed by calling
> >> >+ * rte_hash_free_key_with_position API after all
> >> >+ * the readers have stopped using the hash entry
> >> >+ * corresponding to this key.
> >> >  *
> >> >  * @param h
> >> >  *   Hash table to remove the key from.
> >> >@@ -264,6 +293,8 @@ rte_hash_del_key(const struct rte_hash *h, const
> >> void *key);
> >> >  *   - A positive value that can be used by the caller as an offset into an
> >> >  *     array of user data. This value is unique for this key, and is the same
> >> >  *     value that was returned when the key was added.
> >> >+ *     When lock free concurrency is enabled, this value should be used
> >> >+ *     while calling the rte_hash_free_key_with_position API.
> >> >  */
> >> > int32_t
> >> > rte_hash_del_key_with_hash(const struct rte_hash *h, const void
> >> >*key, hash_sig_t sig); @@ -290,6 +321,30 @@
> >> rte_hash_get_key_with_position(const struct rte_hash *h, const
> >> int32_t position,
> >> > 			       void **key);
> >> >
> >> [Wang, Yipeng] If possible, how about having a new delete function
> >> instead of modifying the current one?
> >> I think it does not need to be tied with the lockless implementation,
> >> it is orthogonal to multi-threading implementation.
> >> people using locks may still want this new deletion behavior.
> >> If people want old behavior, they can call current API, otherwise
> >> they can call the new deletion function, followed by
> Rte_hash_free_key_with_position later.
> >I like the terms 'delete' and 'free'. I am finding it hard to come up
> >with a good name for the API. It will be on the lines of
> 'rte_hash_del_key_with_hash_no_free' - I do not like the name much.
> >Instead, we could have a configuration flag for the hash table,
> >'RTE_HASH_EXTRA_FLAGS_FREE_MEM_ON_DEL'. If this is enabled,
> 'rte_hash_del_...' APIs will free the key store index and any internal memory.
> Enabling lock-free RW concurrency will enable this flag.
> >User can enable this flag explicitly while not using lock-free RW concurrency
> as well.
> [Wang, Yipeng] I am OK with either way. For flag, maybe we should call it
> RTE_HASH_EXTRA_FLAGS_RECYCLE _ON_DEL, since The key-data pair index is
> recycled to be more specific. User should know that the index might be re-
> used by another write.
> BTW, current flag is only 8 bit, as we specify more and more flags, maybe we
> should announce an API change to change it to 32bit for next release.
I agree. Do you know how to do this? Do you want to take care of this?