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From Aaron Davidson <ilike...@gmail.com>
Subject Re: Worker and Nodes
Date Sun, 22 Feb 2015 00:42:24 GMT
Note that the parallelism (i.e., number of partitions) is just an upper
bound on how much of the work can be done in parallel. If you have 200
partitions, then you can divide the work among between 1 and 200 cores and
all resources will remain utilized. If you have more than 200 cores,
though, then some will not be used, so you would want to increase
parallelism further. (There are other rules-of-thumb -- for instance, it's
generally good to have at least 2x more partitions than cores for straggler
mitigation, but these are essentially just optimizations.)

Further note that when you increase the number of Executors for the same
set of resources (i.e., starting 10 Executors on a single machine instead
of 1), you make Spark's job harder. Spark has to communicate in an
all-to-all manner across Executors for shuffle operations, and it uses TCP
sockets to do so whether or not the Executors happen to be on the same
machine. So increasing Executors without increasing physical resources
means Spark has to do more communication to do the same work.

We expect that increasing the number of Executors by a factor of 10, given
an increase in the number of physical resources by the same factor, would
also improve performance by 10x. This is not always the case for the
precise reason above (increased communication overhead), but typically we
can get close. The actual observed improvement is very algorithm-dependent,
though; for instance, some ML algorithms become hard to scale out past a
certain point because the increase in communication overhead outweighs the
increase in parallelism.

On Sat, Feb 21, 2015 at 8:19 AM, Deep Pradhan <pradhandeep1991@gmail.com>
wrote:

> So, if I keep the number of instances constant and increase the degree of
> parallelism in steps, can I expect the performance to increase?
>
> Thank You
>
> On Sat, Feb 21, 2015 at 9:07 PM, Deep Pradhan <pradhandeep1991@gmail.com>
> wrote:
>
>> So, with the increase in the number of worker instances, if I also
>> increase the degree of parallelism, will it make any difference?
>> I can use this model even the other way round right? I can always predict
>> the performance of an app with the increase in number of worker instances,
>> the deterioration in performance, right?
>>
>> Thank You
>>
>> On Sat, Feb 21, 2015 at 8:52 PM, Deep Pradhan <pradhandeep1991@gmail.com>
>> wrote:
>>
>>> Yes, I have decreased the executor memory.
>>> But,if I have to do this, then I have to tweak around with the code
>>> corresponding to each configuration right?
>>>
>>> On Sat, Feb 21, 2015 at 8:47 PM, Sean Owen <sowen@cloudera.com> wrote:
>>>
>>>> "Workers" has a specific meaning in Spark. You are running many on one
>>>> machine? that's possible but not usual.
>>>>
>>>> Each worker's executors have access to a fraction of your machine's
>>>> resources then. If you're not increasing parallelism, maybe you're not
>>>> actually using additional workers, so are using less resource for your
>>>> problem.
>>>>
>>>> Or because the resulting executors are smaller, maybe you're hitting
>>>> GC thrashing in these executors with smaller heaps.
>>>>
>>>> Or if you're not actually configuring the executors to use less
>>>> memory, maybe you're over-committing your RAM and swapping?
>>>>
>>>> Bottom line, you wouldn't use multiple workers on one small standalone
>>>> node. This isn't a good way to estimate performance on a distributed
>>>> cluster either.
>>>>
>>>> On Sat, Feb 21, 2015 at 3:11 PM, Deep Pradhan <
>>>> pradhandeep1991@gmail.com> wrote:
>>>> > No, I just have a single node standalone cluster.
>>>> >
>>>> > I am not tweaking around with the code to increase parallelism. I am
>>>> just
>>>> > running SparkKMeans that is there in Spark-1.0.0
>>>> > I just wanted to know, if this behavior is natural. And if so, what
>>>> causes
>>>> > this?
>>>> >
>>>> > Thank you
>>>> >
>>>> > On Sat, Feb 21, 2015 at 8:32 PM, Sean Owen <sowen@cloudera.com>
>>>> wrote:
>>>> >>
>>>> >> What's your storage like? are you adding worker machines that are
>>>> >> remote from where the data lives? I wonder if it just means you
are
>>>> >> spending more and more time sending the data over the network as
you
>>>> >> try to ship more of it to more remote workers.
>>>> >>
>>>> >> To answer your question, no in general more workers means more
>>>> >> parallelism and therefore faster execution. But that depends on
a lot
>>>> >> of things. For example, if your process isn't parallelize to use
all
>>>> >> available execution slots, adding more slots doesn't do anything.
>>>> >>
>>>> >> On Sat, Feb 21, 2015 at 2:51 PM, Deep Pradhan <
>>>> pradhandeep1991@gmail.com>
>>>> >> wrote:
>>>> >> > Yes, I am talking about standalone single node cluster.
>>>> >> >
>>>> >> > No, I am not increasing parallelism. I just wanted to know
if it is
>>>> >> > natural.
>>>> >> > Does message passing across the workers account for the happenning?
>>>> >> >
>>>> >> > I am running SparkKMeans, just to validate one prediction model.
I
>>>> am
>>>> >> > using
>>>> >> > several data sets. I have a standalone mode. I am varying the
>>>> workers
>>>> >> > from 1
>>>> >> > to 16
>>>> >> >
>>>> >> > On Sat, Feb 21, 2015 at 8:14 PM, Sean Owen <sowen@cloudera.com>
>>>> wrote:
>>>> >> >>
>>>> >> >> I can imagine a few reasons. Adding workers might cause
fewer
>>>> tasks to
>>>> >> >> execute locally (?) So you may be execute more remotely.
>>>> >> >>
>>>> >> >> Are you increasing parallelism? for trivial jobs, chopping
them up
>>>> >> >> further may cause you to pay more overhead of managing
so many
>>>> small
>>>> >> >> tasks, for no speed up in execution time.
>>>> >> >>
>>>> >> >> Can you provide any more specifics though? you haven't
said what
>>>> >> >> you're running, what mode, how many workers, how long it
takes,
>>>> etc.
>>>> >> >>
>>>> >> >> On Sat, Feb 21, 2015 at 2:37 PM, Deep Pradhan
>>>> >> >> <pradhandeep1991@gmail.com>
>>>> >> >> wrote:
>>>> >> >> > Hi,
>>>> >> >> > I have been running some jobs in my local single node
stand
>>>> alone
>>>> >> >> > cluster. I
>>>> >> >> > am varying the worker instances for the same job,
and the time
>>>> taken
>>>> >> >> > for
>>>> >> >> > the
>>>> >> >> > job to complete increases with increase in the number
of
>>>> workers. I
>>>> >> >> > repeated
>>>> >> >> > some experiments varying the number of nodes in a
cluster too
>>>> and the
>>>> >> >> > same
>>>> >> >> > behavior is seen.
>>>> >> >> > Can the idea of worker instances be extrapolated to
the nodes
>>>> in a
>>>> >> >> > cluster?
>>>> >> >> >
>>>> >> >> > Thank You
>>>> >> >
>>>> >> >
>>>> >
>>>> >
>>>>
>>>
>>>
>>
>

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