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From ameet kini <>
Subject Re: number of query threads for batch scanner
Date Wed, 26 Sep 2012 13:19:18 GMT
So I decided to try something different, and changed my splitting policy.
This ended up with more tablets per tablet server. Interestingly, this
bumped up my maximum concurrent scans on that tablet server. With about 19
tablets, I was able to go up to 6 concurrent scans, which ended up using
all my cores - happy! And I didn't change my numQueryThreads parameter from
the already very high number.

But that leaves me wondering whether the maximum number of concurrent scans
on a given tablet server is related to the number of tablets hit by that
scan on the tablet server. If true, that is interesting, and not what I'd
expected. Given  that the underlying files are immutable, I'm not sure why
there can't be, say, 4 concurrent scans on 1 tablet if there were 4 cores
free to host those scans. What I'm seeing, as described above, is I need to
further split my tablet into > 4 tablets in order to have 4 concurrent


On Tue, Sep 25, 2012 at 3:23 PM, ameet kini <> wrote:

> I should also state the not-so-obvious that my Range spans the entire
> range of the four tablets in question.
> Ameet
> On Tue, Sep 25, 2012 at 3:17 PM, ameet kini <> 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
>> On Tue, Sep 25, 2012 at 3:08 PM, William Slacum <
>>> 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 <>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 <>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
>>>>> (i.e., some values for numQueryThreads result in maximum concurrent scan
>>>>> 1, some values result in 2 concurrent scans, etc.). Can someone shed
>>>>> on what is the relationship between numQueryThreads and number of
>>>>> concurrent scans?
>>>>> public BatchScanner createBatchScanner(String tableName,
>>>>>                                        Authorizations authorizations,
>>>>>                                        int numQueryThreads)
>>>>> A follow-on question would be what is general rule of thumb for
>>>>> setting numQueryThreads? Should it be set to the  # of hosted tablets
>>>>> expected to be consumed by that BatchScanner? Should it be the # of tablet
>>>>> servers expected to be hit by that BatchScanner? Something else?
>>>>> Thanks,
>>>>> Ameet

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