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From "Evo Eftimov" <evo.efti...@isecc.com>
Subject RE: SendersBlockedTimer
Date Fri, 26 May 2017 08:23:23 GMT
Hi Henry,

 

The parquet table which is being exported in full via JDBC is about 800 MB compressed as stored
as parquet file and when extracted as CSV or via JDBC ie uncompressed is 2.6 GB

 

I had already tried impala-shell too and it had demonstrated more or less identical performance
as the Cloudera / Simba driver – impala-shell was run with the –B option and its output
redirected to /dev/null for max performance during fetch 

 

If I extract the parquet table to CSV on HDFS with INSERT SELECT from within Impala that results
in a 2.6 GB csv file which if I then download from HDFS with “hdfs dfs –get” it takes
only 30 sec – this is a staggering difference with the performance demonstrated by impala-shell
and the Cloudera/Simba JDBC driver – are these drivers / tools that poorly optimized if
optimized / designed / implemented properly at all ?

 

Ps: I run the JDBC client with JVM with 4GB heap size to minimize any impact of Garbage Collection
  

 

Regards,

Evo

 

From: Henry Robinson [mailto:henry@apache.org] 
Sent: Thursday, May 25, 2017 10:32 PM
To: user@impala.incubator.apache.org
Subject: Re: SendersBlockedTimer

 

 

 

On 25 May 2017 at 12:19, Evo Eftimov <evo.eftimov@isecc.com> wrote:

Hi Henry,

 

I was referring specifically to the EXCHANGE_NODE section of the Coordinator Fragment –
doesn’t that pin it down specifically to the Coordinator Node ie the node to which the JDBC
Client is connected directly ?

 

Also how can the streaming the records from simple full table scan query like “select *
from table” be accelerated so that SendersBlockedTimer  value does not represent the 95%
of the overall time of the query. Basically imagine you have a 3GB parquet table in Impala
and a JDBC Driver Client connected to the Coordinator ImpalaD and trying to stream out all
of the data in the table (3GB) as quickly as possible.

The execution part of the query completes blindingly fast and the data is streamed out of
HDFS within 30 seconds. However the Fetch phase of the full table scan query takes 15 min
as 14 min and 30 sec of that time is  the value in the SendersBlockedTimer 

 

The JDBC Client uses the latest Cloudera JDBC driver for Impala (which is actually the Simba
driver) and performs nothing but just ResultSet.next() ie not parsing and data transformation
of the columns of each row, no output to screen or disk etc. The network between the JDBC
Client and Coordinator is 10 GB and “hdfs client get” of the csv version of the same table
takes only 30 sec …. 

 

Out of the above 15 min total time, Client Fetch Wait Time is 35% or about 6 min. Then we
also have  SendersBlockedTimer of 14 min and 30 sec – so who is to be blamed here for the
slow streaming of records compared to hdfs get – a) innefecient implementation of the JDBC
Client or the Coordinator Node needing more resources like more parallel threads and therefore
CPU cores etc  

 

How do we interpret the above two figures and what do they point to - the jdbc driver or the
Coordinator Node

 

Most likely the driver, as the query takes 6 minutes, per the Client Fetch Wait Time. SendersBlockedTimer
tracks the amount of time for which at least one sender was blocked. Since it is high, we
know that the coordinator is moving slower than the results are being sent to it. The coordinator
does very little in a SELECT * query, so the likelihood is that it is serving rows to the
client as fast as it can consume them. Therefore I'd expect the client to be the bottleneck.


 

Try using the impala-shell, and setting -B (and redirecting the output to /dev/null); this
is about as fast as a single client can go right now and should give you a feeling for a lower
bound on the query performance.

 

How much data does this query return? The client API and driver are not really optimized for
large ETL-style retrieval - for that you might be better off using INSERT to write some files
to HDFS, and then downloading them in parallel from HDFS.

 

Best,

Henry

 

 

Regards,

Evo

 

From: Henry Robinson [mailto:henry@apache.org] 
Sent: Thursday, May 25, 2017 7:23 PM
To: user@impala.incubator.apache.org; evo.eftimov@isecc.com
Subject: Re: SendersBlockedTimer

 

Hi Evo - 

 

Just to clarify: the EXCHANGE_NODE is the operator in the plan tree which mediates communication
between workers, not between the client and the coordinator.

 

The SendersBlockedTimer measures the amount of time that senders have row batches to deliver
to an exchange node, but the exchange is busy delivering a previously sent row batch. That
is, the senders are sending faster than the exchange node (and the upstream plan) processes
those rows. 

 

In a select * from table query, there'll be one exchange on the coordinator, but that's not
generally true - exchanges connect all the fragment instances. Having the senders blocked
in this case is typical, because there'll lots of senders sending at high rate fanning in
to a single receiver, serving a single client.

 

The delivery of rows to the client is managed by the coordinator fragment instance through
a different part of the code to the exchange node. 

 

Henry

 

On 25 May 2017 at 08:31, Evo Eftimov <evo.eftimov@isecc.com> wrote:

What is the purpose of SendersBlockedTimer attribute in the EXCHANGE_NODE section of the Coordinator
Fragment – part of the PROFILE of SQL statement executed by Impala

 

I have reviewed the Impala source code and know that the Exchange Node uses a Blocking Queue
as part of “Stream Manager” module which it instantiates 

 

In the specific context I am interested in, the Exchange Node returns the row from a result
set to a JDBC driver client. The result set is produced by a simple full table scan only query
of the type “select * from table”

 

The “Sender” Parallel Threads (presumably with the Exchange Node) publish rows to the
Blocking Queue also in the Exchange Node and the JDBC client reads rows from the same queue
via remote JDBC session / connection over TCP/IP – is that a correct description of how
the Exchange Node mediates between JDBC client on the one hand and ImpalaD workers on the
other? Btw the Exchange Node is part of the Coordinator Node in terms of terminology – right?

 

My specific question is what is the purpose/meaning  of   SendersBlockedTimer – e.g. does
it mean that the Sender Threads WITHIN the Exchange Node have been in a blocked state for
the time shown in the value of the attribute. And if this is correct then does that mean that
they have been blocked because the JDBC Client couldn’t not keep up with draining the Blocking
Queue during the aggregated time duration in SendersBlockedTimer?

 

 

 


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