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From Ted Yu <yuzhih...@gmail.com>
Subject Re: TableSnapshotInputFormat Behavior In HBase 1.4.0
Date Mon, 12 Mar 2018 16:14:08 GMT
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.mufti@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.mufti@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.mufti@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.mufti@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
> >>>
> >>>
> >>
> >>
> >
>

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