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From "Stephen Allen (Commented) (JIRA)" <>
Subject [jira] [Commented] (JENA-126) Change temporary table threshold policy from count to memory size
Date Fri, 30 Sep 2011 17:21:45 GMT


Stephen Allen commented on JENA-126:

I believe what you describe is what Pig does [1].  It is complicated for a couple of reasons:

1) Hard to reason about which bags to spill.  If you want to do a good job, I think you need:
   1a) Some way to estimate the size of each bag (Pig does this)
   1b) Maybe some way to know which bag is "hot" so as not to spill it.  Some kind of LRU
scheme?  Memory-mapped files would work great here, but now you're talking about off-heap
memory and serialization/deserialization costs. (Pig does not do this)
2) The DataBags classes need to be thread-safe and handle spill requests from the memory management
thread.  Perhaps this can be avoided by calculating it in-line, but testing the free memory
size may be expensive, and then you can only spill bags in your own thread.
3) Need to make sure we spill before the system starts using virtual memory (swap) on its

I think that doing a memory limit per operator may be simpler, since it essentially only requires
you to 1a.  Willing to be proven wrong though, since your idea eliminates the need for a user
configuration option.


> Change temporary table threshold policy from count to memory size
> -----------------------------------------------------------------
>                 Key: JENA-126
>                 URL:
>             Project: Jena
>          Issue Type: Improvement
>          Components: ARQ
>            Reporter: Stephen Allen
> The "workCount" setting for temporary table sizes is not a good configuration option.
 Binding sizes could potentially vary from as little as 32 bytes (8 byte ref to the binding
+ 8 byte ref to a variable + 8 byte nodeID + 8 byte object overhead), to some bindings with
multi-megabyte strings.  Asking the user to know which one it is likely to be, and then how
that count translates into memory usage (the real resource we are attempting to control) is
already way too much IMO.
> OK, so what the user wants is a way to specify the amount of memory that can be used
by each query operator for temporary tables [1][2][3].  Hmm, wait, no what he maybe wants
is a way to specify a the total memory used for temporary tables per query?  No, maybe he
wants to specify it for the whole query engine.
> But that last paragraph is not accurate.  What he *really* wants is a system that answers
all of his queries for whatever data he has as fast as possible.  He doesn't want to have
to configure any parameters.  Unfortunately, this is a really hard dynamic optimization problem
so we foist it off on the user, hoping he'll be able to come up with some value.
> We need to decide on what we want to use as a config parameter.  I believe it should
be a "workMem" or "tmpTableSize" setting that specifies the max memory usage of a temporary
table before it is converted into an on-disk table.
> [1] This is what most DB systems provide, specifically PostgreSQL and MySQL both have
per operator temporary table sizes.  PostgreSQL calls the setting "work_mem" and MySQL calls
it "tmp_table_size"
> [2]
> [3]

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