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Accumulo >> mail # dev >> multi-table isolated batch scanner


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Adam Fuchs 2013-04-15, 19:00
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William Slacum 2013-04-15, 19:06
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Adam Fuchs 2013-04-15, 19:10
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Christopher 2013-04-15, 20:13
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Adam Fuchs 2013-04-15, 21:06
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Keith Turner 2013-04-15, 21:33
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Adam Fuchs 2013-04-15, 22:19
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RE: multi-table isolated batch scanner
---> I have found that increasing the buffer size also increases the latency
for getting the first results.

  We have found that to be true also, we do the opposite to get to the first
result faster. Of course we are not performing a local sort first.

---> increasing the batch size too much puts significant memory requirements
on the process running the batch scanner

  Pushing the problem from the client to the server increases the
complexity. I would be concerned with multiple concurrent scans that are
saving state. The server side state will compete for tserver application
memory. I would assume that you would have to build some feature to restrict
the amount of memory that the state can consume.

-----Original Message-----
From: Adam Fuchs [mailto:[EMAIL PROTECTED]]
Sent: Monday, April 15, 2013 6:19 PM
To: [EMAIL PROTECTED]
Subject: Re: multi-table isolated batch scanner

Keith,

In this case we're filling the buffer before we can amortize the search
cost. We're using a document-partitioned table design and we have to do a
local sort before we can get the first result.

I have found that increasing the buffer size also increases the latency for
getting the first results. This application is both latency and throughput
sensitive. In addition, increasing the batch size too much puts significant
memory requirements on the process running the batch scanner.

Adam

On Mon, Apr 15, 2013 at 5:33 PM, Keith Turner <[EMAIL PROTECTED]> wrote:

> On Mon, Apr 15, 2013 at 5:06 PM, Adam Fuchs <[EMAIL PROTECTED]> wrote:
> > Chris,
> >
> > The desire for isolation stems from the desire to amortize some
> computation
> > over a number of results. Say it takes 5 seconds to compute an
> intersection
>
> Would increasing the size of the key/value buffer help in your case?
> The iterator stack is not torn down until that buffer fills up or the
> end of tablet is reached.  Are you concerned about the cost of
> reconstructing the iterator stack across tablets?
>
> > of a couple of sets within the iterators, and then streaming back
> > the results takes a minute or so. If I have to redo the 5 second
> > computation many times, as in to support the reconstruction of the
> > iterator tree,
> then
> > that computation may start to dominate my query performance.
> > Primarily, this means I need to be able to continue a scan without
> > having to rebuild the iterators. Isolation in the scanner has that
> > side effect. Proper isolation would be a "nice-to-have", but I can deal
with not having it.
> >
> > Adam
> >
> >
> >
> > On Mon, Apr 15, 2013 at 4:13 PM, Christopher <[EMAIL PROTECTED]>
> wrote:
> >
> >> Adam-
> >>
> >> It seems like you're talking about two features at once:
> >> 1) Multi-table batch scanner.
> >> 2) Scan Isolation on batch scanners like we have on regular scanners.
> >> Is that correct?
> >>
> >> I can see the utility of a multi-table batch scanner, but I haven't
> >> seen a compelling need for implementing isolation on the
> >> batch-scanners. Do you have a use case in mind for that?
> >>
> >> Also, it seems that your use case for isolation is not so much the
> >> isolated reads, but the statefulness of the iterator stack on the
> >> server side. Is this correct? If so, I'm even more curious about
> >> your use case for this, since that statefulness is only guaranteed
per-row.
> >>
> >>
> >> --
> >> Christopher L Tubbs II
> >> http://gravatar.com/ctubbsii
> >>
> >>
> >> On Mon, Apr 15, 2013 at 3:10 PM, Adam Fuchs <[EMAIL PROTECTED]> wrote:
> >> > Thanks Bill,
> >> >
> >> > I care about latency and throughput. First available result
> >> > ordering
> is
> >> > fine, though.
> >> >
> >> > Does Guava just chain through a collection of iterators,
> >> > completing
> one
> >> > then moving to the next?
> >> >
> >> > Adam
> >> >
> >> >
> >> >
> >> > On Mon, Apr 15, 2013 at 3:06 PM, William Slacum <
> >> > [EMAIL PROTECTED]> wrote:
> >> >
> >> >> How are you expecting to get results back? Guava's Iterables
tables.
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Adam Fuchs 2013-04-15, 23:01
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Keith Turner 2013-04-16, 13:29
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Adam Fuchs 2013-04-16, 16:33
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Keith Turner 2013-04-15, 20:48