Saad: I encourage you to open an HBase JIRA outlining your use case and the config knobs you added through a patch.
We can see the details for each config and make recommendation accordingly. Thanks On Mon, Mar 12, 2018 at 8:43 AM, Saad Mufti <saad.mu...@gmail.com> wrote: > I have create a company specific branch and added 4 new flags to control > this behavior, these gave us a huge performance boost when running Spark > jobs on snapshots of very large tables in S3. I tried to do everything > cleanly but > > a) not being familiar with the whole test strategies I haven't had time to > add any useful tests, though of course I left the default behavior the > same, and a lot of the behavior I control wit these flags only affect > performance, not the final result, so I would need some pointers on how to > add useful tests > b) I added a new flag to be an overall override for prefetch behavior that > overrides any setting even in the column family descriptor, not sure if > what I did was entirely in the spirit of what HBase does > > Again these if used properly would only impact jobs using > TableSnapshotInputFormat in their Spark or M-R jobs. Would someone from the > core team be willing to look at my patch? I have never done this before, so > would appreciate a quick pointer on how to send a patch and get some quick > feedback. > > Cheers. > > ---- > Saad > > > > On Sat, Mar 10, 2018 at 9:56 PM, Saad Mufti <saad.mu...@gmail.com> wrote: > > > The question remain though of why it is even accessing a column family's > > files that should be excluded based on the Scan. And that column family > > does NOT specify prefetch on open in its schema. Only the one we want to > > read specifies prefetch on open, which we want to override if possible > for > > the Spark job. > > > > ---- > > Saad > > > > > > On Sat, Mar 10, 2018 at 9:51 PM, Saad Mufti <saad.mu...@gmail.com> > wrote: > > > >> See below more I found on item 3. > >> > >> Cheers. > >> > >> ---- > >> Saad > >> > >> On Sat, Mar 10, 2018 at 7:17 PM, Saad Mufti <saad.mu...@gmail.com> > wrote: > >> > >>> Hi, > >>> > >>> I am running a Spark job (Spark 2.2.1) on an EMR cluster in AWS. There > >>> is no Hbase installed on the cluster, only HBase libs linked to my > Spark > >>> app. We are reading the snapshot info from a HBase folder in S3 using > >>> TableSnapshotInputFormat class from HBase 1.4.0 to have the Spark job > read > >>> snapshot info directly from the S3 based filesystem instead of going > >>> through any region server. > >>> > >>> I have observed a few behaviors while debugging performance that are > >>> concerning, some we could mitigate and other I am looking for clarity > on: > >>> > >>> 1) the TableSnapshotInputFormatImpl code is trying to get locality > >>> information for the region splits, for a snapshots with a large number > of > >>> files (over 350000 in our case) this causing single threaded scan of > all > >>> the file listings in a single thread in the driver. And it was useless > >>> because there is really no useful locality information to glean since > all > >>> the files are in S3 and not HDFS. So I was forced to make a copy of > >>> TableSnapshotInputFormatImpl.java in our code and control this with a > >>> config setting I made up. That got rid of the hours long scan, so I am > good > >>> with this part for now. > >>> > >>> 2) I have set a single column family in the Scan that I set on the > hbase > >>> configuration via > >>> > >>> scan.addFamily(str.getBytes())) > >>> > >>> hBaseConf.set(TableInputFormat.SCAN, convertScanToString(scan)) > >>> > >>> > >>> But when this code is executing under Spark and I observe the threads > >>> and logs on Spark executors, I it is reading from S3 files for a column > >>> family that was not included in the scan. This column family was > >>> intentionally excluded because it is much larger than the others and > so we > >>> wanted to avoid the cost. > >>> > >>> Any advice on what I am doing wrong would be appreciated. > >>> > >>> 3) We also explicitly set caching of blocks to false on the scan, > >>> although I see that in TableSnapshotInputFormatImpl.java it is again > >>> set to false internally also. But when running the Spark job, some > >>> executors were taking much longer than others, and when I observe their > >>> threads, I see periodic messages about a few hundred megs of RAM used > by > >>> the block cache, and the thread is sitting there reading data from S3, > and > >>> is occasionally blocked a couple of other threads that have the > >>> "hfile-prefetcher" name in them. Going back to 2) above, they seem to > be > >>> reading the wrong column family, but in this item I am more concerned > about > >>> why they appear to be prefetching blocks and caching them, when the > Scan > >>> object has a setting to not cache blocks at all? > >>> > >> > >> I think I figured out item 3, the column family descriptor for the table > >> in question has prefetch on open set in its schema. Now for the Spark > job, > >> I don't think this serves any useful purpose does it? But I can't see > any > >> way to override it. If these is, I'd appreciate some advice. > >> > > > >> Thanks. > >> > >> > >>> > >>> Thanks in advance for any insights anyone can provide. > >>> > >>> ---- > >>> Saad > >>> > >>> > >> > >> > > >
