From mboxrd@z Thu Jan 1 00:00:00 1970 From: Honnappa Nagarahalli Subject: Re: [PATCH 3/4] hash: fix rw concurrency while moving keys Date: Thu, 4 Oct 2018 03:32:08 +0000 Message-ID: References: <1536253938-192391-1-git-send-email-honnappa.nagarahalli@arm.com> <1536253938-192391-4-git-send-email-honnappa.nagarahalli@arm.com> <20180928082610.GA7592@bricha3-MOBL.ger.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Cc: "De Lara Guarch, Pablo" , "dev@dpdk.org" , "Gavin Hu (Arm Technology China)" , Steve Capper , Ola Liljedahl , nd , "Gobriel, Sameh" , Honnappa Nagarahalli To: "Wang, Yipeng1" , "Van Haaren, Harry" , "Richardson, Bruce" Return-path: Received: from EUR01-VE1-obe.outbound.protection.outlook.com (mail-ve1eur01on0083.outbound.protection.outlook.com [104.47.1.83]) by dpdk.org (Postfix) with ESMTP id A392F5F33 for ; Thu, 4 Oct 2018 05:32:10 +0200 (CEST) In-Reply-To: Content-Language: en-US List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" > > > >> >-----Original Message----- > >> >From: Van Haaren, Harry > >> >> > > > > /** > >> >> > > > > * Add a key to an existing hash table. > >> >> > > > >@@ -222,7 +222,7 @@ rte_hash_add_key(const struct rte_hash > >> >> > > > >*h, const void > >> >> > > *key); > >> >> > > > > * array of user data. This value is unique for this ke= y. > >> >> > > > > */ > >> >> > > > > int32_t > >> >> > > > >-rte_hash_add_key_with_hash(const struct rte_hash *h, const > >> >> > > > >void *key, > >> >> > > hash_sig_t sig); > >> >> > > > >+rte_hash_add_key_with_hash(struct rte_hash *h, const void > >> >> > > > >+*key, > >> >> > > hash_sig_t sig); > >> >> > > > > > >> >> > > > > / > >> >> > > > > >> >> > > > I think the above changes will break ABI by changing the > >> >> > > > parameter > >> >> type? > >> >> > > Other people may know better on this. > >> >> > > > >> >> > > Just removing a const should not change the ABI, I believe, > >> >> > > since the const is just advisory hint to the compiler. Actual > >> >> > > parameter size and count remains unchanged so I don't believe > there is an issue. > >> >> > > [ABI experts, please correct me if I'm wrong on this] > >> >> > > >> >> > > >> >> > [Certainly no ABI expert, but...] > >> >> > > >> >> > I think this is an API break, not ABI break. > >> >> > > >> >> > Given application code as follows, it will fail to compile - > >> >> > even though > >> >> running > >> >> > the new code as a .so wouldn't cause any issues (AFAIK). > >> >> > > >> >> > void do_hash_stuff(const struct rte_hash *h, ...) { > >> >> > /* parameter passed in is const, but updated function > >> >> > prototype is > >> >> non- > >> >> > const */ > >> >> > rte_hash_add_key_with_hash(h, ...); } > >> >> > > >> >> > This means that we can't recompile apps against latest patch > >> >> > without application code changes, if the app was passing a const > >> >> > rte_hash struct > >> >> as > >> >> > the first parameter. > >> >> > > >> >> Agree. Do we need to do anything for this? > >> > > >> >I think we should try to avoid breaking API wherever possible. > >> >If we must, then I suppose we could follow the ABI process of a > >> >deprecation notice. > >> > > >> >From my reading of the versioning docs, it doesn't document this case= : > >> >https://doc.dpdk.org/guides/contributing/versioning.html > >> > > >> >I don't recall a similar situation in DPDK previously - so I suggest > >> >you ask Tech board for input here. > >> > > >> >Hope that helps! -Harry > >> [Wang, Yipeng] > >> Honnappa, how about use a pointer to the counter in the rte_hash > >> struct instead of the counter? Will this avoid API change? > >I think it defeats the purpose of 'const' parameter to the API and provi= des > incorrect information to the user. > >IMO, DPDK should have guidelines on how to handle the API compatibility > breaks. I will send an email to tech board on this. > >We can also solve this by having counters on the bucket. I was planning > >to do this little bit later. I will look at the effort involved and may = be do it > now. > [Wang, Yipeng] > I think with ABI/API change, you might need to announce it one release cy= cle > ahead. >=20 > In the cuckoo switch paper: Scalable, High Performance Ethernet Forwardin= g > with CUCKOOSWITCH it separates the version counter array and the hash > table. You can strike a balance between granularity of the version counte= r and > the cache/memory requirement. > Is it a better way? This will introduce another cache line access. It would be good to stay wit= hin the single cacheline. >=20 > Another consideration is current bucket is 64-byte exactly with the parti= al- > key-hashing. > To add another counter, we need to think about changing certain variables= to > still align cache line. The 'flags' structure member is not being used. I plan to remove that. That= will give us 8B, I will use 4B out of it for the counter.