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Accumulo >> mail # user >> number of query threads for batch scanner


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Re: number of query threads for batch scanner
Thanks Eric and Keith.

Is there any reason why the number of concurrent scans on a given tablet
server depends on the number of tablets and not the number of cores on that
tablet server? I'm looking at TabletServerBatchReaderIterator.doLookups.

Take Keith's example:

 * For 1000 ranges that map to 1 tablet, it will execute 1 concurrent scan.

Say, I had 8 cores on that tablet server and my tablet is large enough to
warrant 8 concurrent scans. Sure, I can go about and further split my
tablet, and get 8 concurrent scans - I ended up doing that. But is there
any reason why 8 concurrent scans can't go against a single tablet? Maybe
its difficult to estimate benefits of parallelism at that level, and its
best left to users to tune the number of tablets, and base the level of
parallelism on the number of tablets?

Btw, the shell utility "merge -s <size>" rocks :)

Thanks,
Ameet
On Fri, Sep 28, 2012 at 8:04 AM, Keith Turner <[EMAIL PROTECTED]> wrote:

> On Tue, Sep 25, 2012 at 3:17 PM, ameet kini <[EMAIL PROTECTED]> wrote:
> > Thanks William.
> >
> > The issue here is that without knowing how the numQueryThreads
> translates to
> > the number of concurrent scans, I cannot effectively tune that parameter
> to
> > maximize resource usage on the tablet server. What I'm seeing is that
> even
> > though there are four tablets on the tablet server, my number of
> concurrent
> > scans never exceeds 3. This is despite setting numQueryThreads to a very
> > high number and having 8 cores on the tablet server. I suspect with 3
> > concurrent scans and no garbage collection happening at that moment,
> most of
> > the cores are sitting idle.
> >
> > Ameet
>
> The amount if parallelism is determined by how your ranges map to
> tablets. Below are some examples.
>
>  * For one range that maps to 10 tablets on 10 tablets severs, it will
> execute 10 concurrent scans if numQueryThreads is >= 10.
>  * For 1000 ranges that map to 10 tablets on 10 tablet servers, it
> will execute 10 concurrent scans if numQueryThreads is >= 10.
>  * For 1000 ranges that map to 10 tablets on 10 tablet servers, it
> will execute 5 concurrent scans if numQueryThreads is 5.
>  * For 1000 ranges that map to 1 tablet, it will execute 1 concurrent scan.
>
> If you have more query threads than tablet server, the client code
> will try to execute concurrent scans on a single tablet server.
>
> You can look at TabletServerBatchReaderIterator.doLookups() for the
> details.  In this method it creates QueryTask objects and places them
> on a thread pool.  The size of the thread pool is the user specified
> numQueryThreads.
>
> >
> > On Tue, Sep 25, 2012 at 3:08 PM, William Slacum
> > <[EMAIL PROTECTED]> wrote:
> >>
> >> It should really be dependent upon the resources available to the
> client.
> >> You can set an arbitrarily high number of threads, but you're still
> bound by
> >> the number of parallel operations the CPU can make. I would assume the
> sweet
> >> spot is somewhere around that number-- try doing a small bench mark
> with 2,
> >> 4, 8, 16, etc threads and see where your performance starts to level
> off.
> >>
> >>
> >> On Tue, Sep 25, 2012 at 11:45 AM, ameet kini <[EMAIL PROTECTED]>
> wrote:
> >>>
> >>> Probably worth adding that the table mentioned below has a bunch of
> >>> tablets on other tablet servers as well, which is why I'm using
> >>> BatchScanner. I'm just not sure how the numQueryThreads relates to the
> >>> number of a concurrent scans on a given tablet server.
> >>>
> >>> Thanks
> >>>
> >>>
> >>> On Tue, Sep 25, 2012 at 2:22 PM, ameet kini <[EMAIL PROTECTED]>
> wrote:
> >>>>
> >>>>
> >>>> I have a table with 4 tablets on a given tablet server. Depending on
> the
> >>>> numQueryThreads parameter below, I see a varying number of maximum
> >>>> concurrent scans on that table. This maximum number varies from 1 to 3
> >>>> (i.e., some values for numQueryThreads result in maximum concurrent
> scan of
> >>>> 1, some values result in 2 concurrent scans, etc.). Can someone shed