On Thu, 18 Aug 2016, Haomai Wang wrote: > On Thu, Aug 18, 2016 at 12:10 AM, Sage Weil <sweil@xxxxxxxxxx> wrote: > > On Thu, 18 Aug 2016, Haomai Wang wrote: > >> On Wed, Aug 17, 2016 at 11:43 PM, Sage Weil <sweil@xxxxxxxxxx> wrote: > >> > On Wed, 17 Aug 2016, Haomai Wang wrote: > >> >> On Wed, Aug 17, 2016 at 11:25 PM, Sage Weil <sweil@xxxxxxxxxx> wrote: > >> >> > On Wed, 17 Aug 2016, Haomai Wang wrote: > >> >> >> another latency perf problem: > >> >> >> > >> >> >> rocksdb log is on bluefs and mainly uses append and fsync interface to > >> >> >> complete WAL. > >> >> >> > >> >> >> I found the latency between kv transaction submitting isn't negligible > >> >> >> and limit the transaction throughput. > >> >> >> > >> >> >> So what if we implement a async transaction submit in rocksdb side > >> >> >> using callback way? It will decrease kv in queue latency. It would > >> >> >> help rocksdb WAL performance close to FileJournal. And async interface > >> >> >> will help control each kv transaction size and make transaction > >> >> >> complete smoothly instead of tps spike with us precious. > >> >> > > >> >> > Can we get the same benefit by calling BlueFS::_flush on the log whenever > >> >> > we have X bytes accumulated (I think there is an option in rocksdb that > >> >> > drives this already, actually)? Changing the interfaces around will > >> >> > change the threading model (= work) but doesn't actually change who needs > >> >> > to wait and when. > >> >> > >> >> why we need to wait after interface change? > >> >> > >> >> 1. kv thread submit transaction with callback. > >> >> 2. rocksdb append and call bluefs aio_submit with callback > >> >> 3. bluefs submit aio write with callback > >> >> 4. KernelDevice will poll linux aio event and execute callback inline > >> >> or queue finish > >> >> 5. callback will notify we complete the kv transaction > >> >> > >> >> the main task is implement logics in rocksdb log*.cc and bluefs aio > >> >> submit interface.... > >> >> > >> >> Is anything I'm missing? > >> > > >> > That can all be done with callbacks, but even if we do the kv thread will > >> > still need to wait on the callback before doing anything else. > >> > > >> > Oh, you're suggesting we have multiple batches of transactions in flight. > >> > Got it. > >> > >> I don't think so.. because bluefs has lock for fsync and flush. So > >> multi rocksdb thread will be serial to flush... > > > > Oh, this was fixed recently: > > > > 10d055d65727e47deae4e459bc21aaa243c24a7d > > 97699334acd59e9530d36b13d3a8408cabf848ef > > Hmm, looks better! > > The only thing is I notice we don't have FileWriter lock for "buffer", > so multi rocksdb writer will result in corrupt? I haven't look at > rocksdb to check, but I think if posix backend, rocksdb don't need to > have a look to protect log append racing. Hmm, there is this option: https://github.com/ceph/ceph/blob/master/src/os/bluestore/BlueRocksEnv.cc#L224 but that doesn't say anything about more than one concurrent Append. You're probably right and we need some extra locking here... sage > > > > >> and another thing is the single thread is help for polling case..... > >> from my current perf, compared queue filejournal class, rocksdb plays > >> 1.5x-2x latency, in heavy load it will be more .... Yes, filejournal > >> exactly has a good pipeline for pure linux aio job. > > > > Yeah, I think you're right. Even if we do the parallel submission, we > > don't want to do parallel blocking (since the callers don't want to > > block), so we'll still want async completion/notification of commit. > > > > No idea if this is something the rocksdb folks are already interested in > > or not... want to ask them on their cool facebook group? :) > > > > https://www.facebook.com/groups/rocksdb.dev/ > > sure > > > > > sage > > > > > >> > >> > > >> > I think we will get some of the benefit by enabling the parallel > >> > transaction submits (so we don't funnel everything through > >> > _kv_sync_thread). I think we should get that merged first and see how it > >> > behaves before taking the next step. I forgot to ask Varada is standup > >> > this morning what the current status of that is. Varada? > >> > > >> > sage > >> > > >> >> > >> >> > > >> >> > sage > >> >> > > >> >> > > >> >> > > >> >> >> > >> >> >> > >> >> >> On Wed, Aug 17, 2016 at 10:26 PM, Sage Weil <sweil@xxxxxxxxxx> wrote: > >> >> >> > I think we need to look at other changes in addition to the encoding > >> >> >> > performance improvements. Even if they end up being good enough, these > >> >> >> > changes are somewhat orthogonal and at least one of them should give us > >> >> >> > something that is even faster. > >> >> >> > > >> >> >> > 1. I mentioned this before, but we should keep the encoding > >> >> >> > bluestore_blob_t around when we load the blob map. If it's not changed, > >> >> >> > don't reencode it. There are no blockers for implementing this currently. > >> >> >> > It may be difficult to ensure the blobs are properly marked dirty... I'll > >> >> >> > see if we can use proper accessors for the blob to enforce this at compile > >> >> >> > time. We should do that anyway. > >> >> >> > > >> >> >> > 2. This turns the blob Put into rocksdb into two memcpy stages: one to > >> >> >> > assemble the bufferlist (lots of bufferptrs to each untouched blob) > >> >> >> > into a single rocksdb::Slice, and another memcpy somewhere inside > >> >> >> > rocksdb to copy this into the write buffer. We could extend the > >> >> >> > rocksdb interface to take an iovec so that the first memcpy isn't needed > >> >> >> > (and rocksdb will instead iterate over our buffers and copy them directly > >> >> >> > into its write buffer). This is probably a pretty small piece of the > >> >> >> > overall time... should verify with a profiler before investing too much > >> >> >> > effort here. > >> >> >> > > >> >> >> > 3. Even if we do the above, we're still setting a big (~4k or more?) key > >> >> >> > into rocksdb every time we touch an object, even when a tiny amount of > >> >> >> > metadata is getting changed. This is a consequence of embedding all of > >> >> >> > the blobs into the onode (or bnode). That seemed like a good idea early > >> >> >> > on when they were tiny (i.e., just an extent), but now I'm not so sure. I > >> >> >> > see a couple of different options: > >> >> >> > > >> >> >> > a) Store each blob as ($onode_key+$blobid). When we load the onode, load > >> >> >> > the blobs too. They will hopefully be sequential in rocksdb (or > >> >> >> > definitely sequential in zs). Probably go back to using an iterator. > >> >> >> > > >> >> >> > b) Go all in on the "bnode" like concept. Assign blob ids so that they > >> >> >> > are unique for any given hash value. Then store the blobs as > >> >> >> > $shard.$poolid.$hash.$blobid (i.e., where the bnode is now). Then when > >> >> >> > clone happens there is no onode->bnode migration magic happening--we've > >> >> >> > already committed to storing blobs in separate keys. When we load the > >> >> >> > onode, keep the conditional bnode loading we already have.. but when the > >> >> >> > bnode is loaded load up all the blobs for the hash key. (Okay, we could > >> >> >> > fault in blobs individually, but that code will be more complicated.) > >> >> >> > > >> >> >> > In both these cases, a write will dirty the onode (which is back to being > >> >> >> > pretty small.. just xattrs and the lextent map) and 1-3 blobs (also now > >> >> >> > small keys). Updates will generate much lower metadata write traffic, > >> >> >> > which'll reduce media wear and compaction overhead. The cost is that > >> >> >> > operations (e.g., reads) that have to fault in an onode are now fetching > >> >> >> > several nearby keys instead of a single key. > >> >> >> > > >> >> >> > > >> >> >> > #1 and #2 are completely orthogonal to any encoding efficiency > >> >> >> > improvements we make. And #1 is simple... I plan to implement this > >> >> >> > shortly. > >> >> >> > > >> >> >> > #3 is balancing (re)encoding efficiency against the cost of separate keys, > >> >> >> > and that tradeoff will change as encoding efficiency changes, so it'll be > >> >> >> > difficult to properly evaluate without knowing where we'll land with the > >> >> >> > (re)encode times. I think it's a design decision made early on that is > >> >> >> > worth revisiting, though! > >> >> >> > > >> >> >> > sage > >> >> >> > -- > >> >> >> > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > >> >> >> > the body of a message to majordomo@xxxxxxxxxxxxxxx > >> >> >> > More majordomo info at http://vger.kernel.org/majordomo-info.html > >> >> >> -- > >> >> >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > >> >> >> the body of a message to majordomo@xxxxxxxxxxxxxxx > >> >> >> More majordomo info at http://vger.kernel.org/majordomo-info.html > >> >> >> > >> >> >> > >> >> > >> >> > >> -- > >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > >> the body of a message to majordomo@xxxxxxxxxxxxxxx > >> More majordomo info at http://vger.kernel.org/majordomo-info.html > >> > >> > > -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
