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From Jacques Nadeau <jacq...@dremio.com>
Subject Re: [DISCUSS] Ideas to improve metadata cache read performance
Date Wed, 04 Nov 2015 16:31:35 GMT
I've been thinking more about this and I think Aman's suggestion of Parquet
files is worth a poc.

What we could do:

Run a select * order by partCol1, partCol2, ... , partColN query against
the existing large json partition file and create a new Parquet version of
the file.
Hand write a partition type read against the Parquet APIs using the filter
APIs and see what performance looks like.

Thoughts?

--
Jacques Nadeau
CTO and Co-Founder, Dremio

On Fri, Oct 30, 2015 at 3:36 PM, Parth Chandra <parthc@apache.org> wrote:

> Thanks Steven for the link.
> Your suggestion of storing only the single valued columns is a good one.
> It might be OK to have some of the count* queries run a little slower as
> reading the cache itself is taking way to long.  I'm also looking at
> squashing the column datatype info as there is a lot of redundancy there.
>
>
>
>
> On Fri, Oct 30, 2015 at 3:22 PM, Steven Phillips <steven@dremio.com>
> wrote:
>
> > My view on storing it in some other format is that, yes, it will probably
> > reduce the size of the file, but if we gzip the json file, it should be
> > pretty compact. As for deserialization cost, other formats would be
> faster,
> > but not dramatically faster. Certainly not the order of magnitude faster
> > that we really need it to be. The reason we chose JSON was because it is
> > readable and easier to deal with.
> >
> > As for the old code, I can point you at a branch, but it's probably not
> > very helpful. Unless we want to essentially disable value-based partition
> > pruning when using the cache, the old code will not work.
> >
> > My recommendation would be to come up with a new version of the format
> > which stores only the name and value of columns which are single-valued
> for
> > each file or row group. This will allow partition pruning to work, but
> some
> > count queries may not be as fast any more, because the cache won't have
> > column value counts on a per-rowgroup basis any more.
> >
> > Anyway, here is the link to the original branch.
> >
> > https://github.com/StevenMPhillips/drill/tree/meta
> >
> > On Fri, Oct 30, 2015 at 3:01 PM, Parth Chandra <parthc@apache.org>
> wrote:
> >
> > > Hey Jacques, Steven,
> > >
> > >   Do we have a branch somewhere which has the initial prototype code?
> I'd
> > > like to prune the file a bit as it looks like reducing the size of the
> > > metadata cache file might yield the best results.
> > >
> > >   Also, did we have a particular reason for going with JSON as opposed
> > to a
> > > more compact binary format? Are there any arguments against saving this
> > as
> > > a protobuf/BSON/Parquet file?
> > >
> > > Parth
> > >
> > > On Mon, Oct 26, 2015 at 2:42 PM, Jacques Nadeau <jacques@dremio.com>
> > > wrote:
> > >
> > > > My first thought is we've gotten too generous in what we're storing
> in
> > > the
> > > > Parquet metadata file. Early implementations were very lean and it
> > seems
> > > > far larger today. For example, early implementations didn't keep
> > > statistics
> > > > and ignored row groups (files, schema and block locations only). If
> we
> > > need
> > > > multiple levels of information, we may want to stagger (or normalize)
> > > them
> > > > in the file. Also, we may think about what is the minimum that must
> be
> > > done
> > > > in planning. We could do the file pruning at execution time rather
> than
> > > > single-tracking these things (makes stats harder though).
> > > >
> > > > I also think we should be cautious around jumping to a conclusion
> until
> > > > DRILL-3973 provides more insight.
> > > >
> > > > In terms of caching, I'd be more inclined to rely on file system
> > caching
> > > > and make sure serialization/deserialization is as efficient as
> possible
> > > as
> > > > opposed to implementing an application-level cache. (We already have
> > > enough
> > > > problems managing memory without having to figure out when we should
> > > drop a
> > > > metadata cache :D).
> > > >
> > > > Aside, I always liked this post for entertainment and the thoughts on
> > > > virtual memory:
> https://www.varnish-cache.org/trac/wiki/ArchitectNotes
> > > >
> > > >
> > > > --
> > > > Jacques Nadeau
> > > > CTO and Co-Founder, Dremio
> > > >
> > > > On Mon, Oct 26, 2015 at 2:25 PM, Hanifi Gunes <hgunes@maprtech.com>
> > > wrote:
> > > >
> > > > > One more thing, for workloads running queries over subsets of same
> > > > parquet
> > > > > files, we can consider maintaining an in-memory cache as well.
> > Assuming
> > > > > metadata memory footprint per file is low and parquet files are
> > static,
> > > > not
> > > > > needing us to invalidate the cache often.
> > > > >
> > > > > H+
> > > > >
> > > > > On Mon, Oct 26, 2015 at 2:10 PM, Hanifi Gunes <hgunes@maprtech.com
> >
> > > > wrote:
> > > > >
> > > > > > I am not familiar with the contents of metadata stored but if
> > > > > > deserialization workload seems to be fitting to any of
> > afterburner's
> > > > > > claimed improvement points [1] It could well be worth trying
> given
> > > the
> > > > > > claimed gain on throughput is substantial.
> > > > > >
> > > > > > It could also be a good idea to partition caching over a number
> of
> > > > files
> > > > > > for better parallelization given number of cache files generated
> is
> > > > > > *significantly* less than number of parquet files. Maintaining
> > global
> > > > > > statistics seems an improvement point too.
> > > > > >
> > > > > >
> > > > > > -H+
> > > > > >
> > > > > > 1:
> > > > > >
> > > > >
> > > >
> > >
> >
> https://github.com/FasterXML/jackson-module-afterburner#what-is-optimized
> > > > > >
> > > > > > On Sun, Oct 25, 2015 at 9:33 AM, Aman Sinha <
> amansinha@apache.org>
> > > > > wrote:
> > > > > >
> > > > > >> Forgot to include the link for Jackson's AfterBurner module:
> > > > > >>   https://github.com/FasterXML/jackson-module-afterburner
> > > > > >>
> > > > > >> On Sun, Oct 25, 2015 at 9:28 AM, Aman Sinha <
> amansinha@apache.org
> > >
> > > > > wrote:
> > > > > >>
> > > > > >> > I was going to file an enhancement JIRA but thought
I will
> > discuss
> > > > > here
> > > > > >> > first:
> > > > > >> >
> > > > > >> > The parquet metadata cache file is a JSON file that
contains a
> > > > subset
> > > > > of
> > > > > >> > the metadata extracted from the parquet files.  The
cache file
> > can
> > > > get
> > > > > >> > really large .. a few GBs for a few hundred thousand
files.
> > > > > >> > I have filed a separate JIRA: DRILL-3973 for profiling
the
> > various
> > > > > >> aspects
> > > > > >> > of planning including metadata operations.  In the
meantime,
> the
> > > > > >> timestamps
> > > > > >> > in the drillbit.log output indicate a large chunk of
time
> spent
> > in
> > > > > >> creating
> > > > > >> > the drill table to begin with, which indicates bottleneck
in
> > > reading
> > > > > the
> > > > > >> > metadata.  (I can provide performance numbers later
once we
> > > confirm
> > > > > >> through
> > > > > >> > profiling).
> > > > > >> >
> > > > > >> > A few thoughts around improvements:
> > > > > >> >  - The jackson deserialization of the JSON file is
very slow..
> > can
> > > > > this
> > > > > >> be
> > > > > >> > speeded up ? .. for instance the AfterBurner module
of jackson
> > > > claims
> > > > > to
> > > > > >> > improve performance by 30-40% by avoiding the use of
> reflection.
> > > > > >> >  - The cache file read is a single threaded process.
 If we
> were
> > > > > >> directly
> > > > > >> > reading from parquet files, we use a default of 16
threads.
> > What
> > > > can
> > > > > be
> > > > > >> > done to parallelize the read ?
> > > > > >> >  - Any operation that can be done one time during the
REFRESH
> > > > METADATA
> > > > > >> > command ?  for instance..examining the min/max values
to
> > determine
> > > > > >> > single-value for partition column could be eliminated
if we do
> > > this
> > > > > >> > computation during REFRESH METADATA command and store
the
> > summary
> > > > one
> > > > > >> time.
> > > > > >> >
> > > > > >> >  - A pertinent question is: should the cache file be
stored
> in a
> > > > more
> > > > > >> > efficient format such as Parquet instead of JSON ?
> > > > > >> >
> > > > > >> > Aman
> > > > > >> >
> > > > > >> >
> > > > > >>
> > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>

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