[jira] [Commented] (IMPALA-6678) Better estimate of per-column compressed data size for low-NDV columns.

["ASF subversion and git services (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/IMPALA-6678?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16457854#comment-16457854 ] 

ASF subversion and git services commented on IMPALA-6678:
---------------------------------------------------------

Commit 418c705787f060afb3e9e5fbeadb891b2484b6c5 in impala's branch refs/heads/master from [~tarmstrong@cloudera.com]
[ https://git-wip-us.apache.org/repos/asf?p=impala.git;h=418c705 ]

IMPALA-6679,IMPALA-6678: reduce scan reservation

This has two related changes.

IMPALA-6679: defer scanner reservation increases
------------------------------------------------
When starting each scan range, check to see how big the initial scan
range is (the full thing for row-based formats, the footer for
Parquet) and determine whether more reservation would be useful.

For Parquet, base the ideal reservation on the actual column layout
of each file. This avoids reserving memory that we won't use for
the actual files that we're scanning. This also avoid the need to
estimate ideal reservation in the planner.

We also release scanner thread reservations above the minimum as
soon as threads complete, so that resources can be released slightly
earlier.

IMPALA-6678: estimate Parquet column size for reservation
---------------------------------------------------------
This change also reduces reservation computed by the planner in certain
cases by estimating the on-disk size of column data based on stats. It
also reduces the default per-column reservation to 4MB since it appears
that < 8MB columns are generally common in practice and the method for
estimating column size is biased towards over-estimating. There are two
main cases to consider for the performance implications:
* Memory is available to improve query perf - if we underestimate, we
  can increase the reservation so we can do "efficient" 8MB I/Os for
  large columns.
* The ideal reservation is not available - query performance is affected
  because we can't overlap I/O and compute as much and may do smaller
  (probably 4MB I/Os). However, we should avoid pathological behaviour
  like tiny I/Os.

When stats are not available, we just default to reserving 4MB per
column, which typically is more memory than required. When stats are
available, the memory required can be reduced below when some heuristic
tell us with high confidence that the column data for most or all files
is smaller than 4MB.

The stats-based heuristic could reduce scan performance if both the
conservative heuristics significantly underestimate the column size
and memory is constrained such that we can't increase the scan
reservation at runtime (in which case the memory might be used by
a different operator or scanner thread).

Observability:
Added counters to track when threads were not spawned due to reservation
and to track when reservation increases are requested and denied. These
allow determining if performance may have been affected by memory
availability.

Testing:
Updated test_mem_usage_scaling.py memory requirements and added steps
to regenerate the requirements. Loops test for a while to flush out
flakiness.

Added targeted planner and query tests for reservation calculations and
increases.

Change-Id: Ifc80e05118a9eef72cac8e2308418122e3ee0842
Reviewed-on: http://gerrit.cloudera.org:8080/9757
Reviewed-by: Tim Armstrong <ta...@cloudera.com>
Tested-by: Impala Public Jenkins <im...@cloudera.com>


> Better estimate of per-column compressed data size for low-NDV columns.
> -----------------------------------------------------------------------
>
>                 Key: IMPALA-6678
>                 URL: https://issues.apache.org/jira/browse/IMPALA-6678
>             Project: IMPALA
>          Issue Type: Sub-task
>          Components: Backend
>    Affects Versions: Not Applicable
>            Reporter: Tim Armstrong
>            Assignee: Tim Armstrong
>            Priority: Major
>              Labels: resource-management
>
> In the previous IMPALA-4835 patch, we assumed that the "ideal" memory per Parquet column was 3 * 8MB, except when the total size of the file capped the total amount of memory we might use. This is often an overestimate, particular for smaller files, files with large numbers of columns, and highly compressible data.
> We could do something smarter for Parquet given file sizes, per-partition row count, and column NDV. We can estimate row count per file by dividing the row count by the file size and estimate bytes per value with two methods:
> * For fixed width types, estimating bytes per value based on the type width. We don't know what the physical parquet type is necessarily, but it seems reasonable to estimate based on the type declared in the table.
> * log2(ndv) / 8, assuming that dictionary compression or general-purpose compression will kick in.
> See https://docs.google.com/document/d/1kR0zfevNNUJom3sH1XmposacVZ-QALan7NSwnR5CkSA/edit#heading=h.a2b8e8h5a6en for some analysis. 
> I looked at encoded lineitem data and saw that many of the scanned columns were 3-4MB in size and that we could have estimated an ideal size < 24MB per column based on the above heuristics.



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)

---------------------------------------------------------------------
To unsubscribe, e-mail: issues-all-unsubscribe@impala.apache.org
For additional commands, e-mail: issues-all-help@impala.apache.org