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From Michael Slavitch <slavi...@gmail.com>
Subject Re: Eliminating shuffle write and spill disk IO reads/writes in Spark
Date Fri, 01 Apr 2016 22:28:16 GMT
RAMdisk is a fine interim step but there is a lot of layers eliminated by
keeping things in memory unless there is need for spillover.   At one time
there was support for turning off spilling.  That was eliminated.  Why?

On Fri, Apr 1, 2016, 6:05 PM Mridul Muralidharan <mridul@gmail.com> wrote:

> I think Reynold's suggestion of using ram disk would be a good way to
> test if these are the bottlenecks or something else is.
> For most practical purposes, pointing local dir to ramdisk should
> effectively give you 'similar' performance as shuffling from memory.
>
> Are there concerns with taking that approach to test ? (I dont see
> any, but I am not sure if I missed something).
>
>
> Regards,
> Mridul
>
>
>
>
> On Fri, Apr 1, 2016 at 2:10 PM, Michael Slavitch <slavitch@gmail.com>
> wrote:
> > I totally disagree that it’s not a problem.
> >
> > - Network fetch throughput on 40G Ethernet exceeds the throughput of NVME
> > drives.
> > - What Spark is depending on is Linux’s IO cache as an effective buffer
> pool
> > This is fine for small jobs but not for jobs with datasets in the TB/node
> > range.
> > - On larger jobs flushing the cache causes Linux to block.
> > - On a modern 56-hyperthread 2-socket host the latency caused by multiple
> > executors writing out to disk increases greatly.
> >
> > I thought the whole point of Spark was in-memory computing?  It’s in fact
> > in-memory for some things but  use spark.local.dir as a buffer pool of
> > others.
> >
> > Hence, the performance of  Spark is gated by the performance of
> > spark.local.dir, even on large memory systems.
> >
> > "Currently it is not possible to not write shuffle files to disk.”
> >
> > What changes >would< make it possible?
> >
> > The only one that seems possible is to clone the shuffle service and
> make it
> > in-memory.
> >
> >
> >
> >
> >
> > On Apr 1, 2016, at 4:57 PM, Reynold Xin <rxin@databricks.com> wrote:
> >
> > spark.shuffle.spill actually has nothing to do with whether we write
> shuffle
> > files to disk. Currently it is not possible to not write shuffle files to
> > disk, and typically it is not a problem because the network fetch
> throughput
> > is lower than what disks can sustain. In most cases, especially with
> SSDs,
> > there is little difference between putting all of those in memory and on
> > disk.
> >
> > However, it is becoming more common to run Spark on a few number of beefy
> > nodes (e.g. 2 nodes each with 1TB of RAM). We do want to look into
> improving
> > performance for those. Meantime, you can setup local ramdisks on each
> node
> > for shuffle writes.
> >
> >
> >
> > On Fri, Apr 1, 2016 at 11:32 AM, Michael Slavitch <slavitch@gmail.com>
> > wrote:
> >>
> >> Hello;
> >>
> >> I’m working on spark with very large memory systems (2TB+) and notice
> that
> >> Spark spills to disk in shuffle.  Is there a way to force spark to stay
> in
> >> memory when doing shuffle operations?   The goal is to keep the shuffle
> data
> >> either in the heap or in off-heap memory (in 1.6.x) and never touch the
> IO
> >> subsystem.  I am willing to have the job fail if it runs out of RAM.
> >>
> >> spark.shuffle.spill true  is deprecated in 1.6 and does not work in
> >> Tungsten sort in 1.5.x
> >>
> >> "WARN UnsafeShuffleManager: spark.shuffle.spill was set to false, but
> this
> >> is ignored by the tungsten-sort shuffle manager; its optimized shuffles
> will
> >> continue to spill to disk when necessary.”
> >>
> >> If this is impossible via configuration changes what code changes would
> be
> >> needed to accomplish this?
> >>
> >>
> >>
> >>
> >>
> >> ---------------------------------------------------------------------
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> >>
> >
> >
>
-- 
Michael Slavitch
62 Renfrew Ave.
Ottawa Ontario
K1S 1Z5

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