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. > >> 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
