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From Dylan Hutchison <dhutc...@mit.edu>
Subject Re: Scanning with many singleton ranges?
Date Fri, 03 Apr 2015 03:49:59 GMT
I think it is an OOME.  Here's the debug log file, showing a clear descend
from 189MB free to 52kB free memory before manually restarting the tserver
4 minutes later.  Looks like I lost the .err files for now; would need to
reproduce the crash to get them again.

2015-03-26 08:34:01,242 [tserver.TabletServer] DEBUG: gc
ParNew=26.24(+0.01) secs ConcurrentMarkSweep=0.13(+0.00) secs*
freemem=189,300,488(-330,224) *totalmem=259,522,560
2015-03-26 08:34:01,549 [tserver.TabletServer] DEBUG: ScanSess tid
127.0.0.1:55823 6r 374,161 entries in 2.98 secs, nbTimes = [1 69 3.27 375]
2015-03-26 08:34:01,842 [Audit   ] INFO : operation: permitted; user: root;
client: 127.0.0.1:55823;
2015-03-26 08:34:01,842 [Audit   ] INFO : operation: permitted; user: root;
client: 127.0.0.1:55823;
2015-03-26 08:34:01,844 [tserver.TabletServer] DEBUG: ScanSess tid
127.0.0.1:55823 !0 5 entries in 0.00 secs, nbTimes = [1 1 1.00 1]
2015-03-26 08:34:03,034 [tserver.TabletServer] DEBUG: Got getScans message
from user: !SYSTEM
2015-03-26 08:34:03,091 [tserver.TabletServer] DEBUG: MultiScanSess
127.0.0.1:38998 2 entries in 0.00 secs (lookup_time:0.00 secs tablets:1
ranges:1)
2015-03-26 08:34:04,507 [tserver.TabletServer] DEBUG: gc
ParNew=26.38(+0.14) secs ConcurrentMarkSweep=0.99(+0.86) secs
*freemem=44,246,264(-145,384,448)
*totalmem=259,522,560
2015-03-26 08:34:05,963 [tserver.TabletServer] DEBUG: ScanSess tid
127.0.0.1:55017 !0 0 entries in 0.00 secs, nbTimes = [2 2 2.00 1]
2015-03-26 08:34:05,966 [tserver.TabletServer] DEBUG: gc
ParNew=26.38(+0.00) secs ConcurrentMarkSweep=2.25(+1.26) secs
*freemem=6,657,016(-182,973,696)
*totalmem=259,522,560
2015-03-26 08:34:07,549 [tserver.TabletServer] DEBUG: gc
ParNew=26.38(+0.00) secs ConcurrentMarkSweep=3.73(+1.48) secs
*freemem=439,152(-189,191,560)
*totalmem=259,522,560
2015-03-26 08:34:08,284 [tserver.TabletServer] DEBUG: Got getScans message
from user: !SYSTEM
*2015-03-26 08:34:10,469 [tserver.TabletServer] WARN : Running low on
memory*
2015-03-26 08:34:10,470 [tserver.TabletServer] DEBUG: gc
ParNew=26.38(+0.00) secs ConcurrentMarkSweep=6.63(+2.90) secs
*freemem=52,816(-189,577,896)
*totalmem=259,522,560
2015-03-26 08:34:14,623 [tserver.TabletServer] DEBUG: Got getScans message
from user: !SYSTEM
2015-03-26 08:34:17,382 [tserver.TabletServer] DEBUG: ScanSess tid
127.0.0.1:55017 !0 0 entries in 5.04 secs, nbTimes = [4,972 4,972 4,972.00
1]
2015-03-26 08:34:24,674 [tserver.TabletServer] DEBUG: Got getScans message
from user: !SYSTEM
2015-03-26 08:34:35,716 [cache.LruBlockCache] DEBUG: Cache Stats: Sizes:
Total=23.286858MB (24418040), Free=6.7131424MB (7039240), Max=30.0MB
(31457280), Counts: Blocks=7750, Access=125628, Hit=102578, Miss=23050,
Evictions=25, Evicted=15299, Ratios: Hit Ratio=81.65218234062195%, Miss
Ratio=18.34782063961029%, Evicted/Run=611.9600219726562, Duplicate Reads=1
*2015-03-26 08:38:37,256 [server.Accumulo] INFO : tserver starting*





On Thu, Apr 2, 2015 at 6:34 PM, Josh Elser <josh.elser@gmail.com> wrote:

> That seems perfectly reasonable to me, IMO. I'm surprised to hear the
> tserver crashed.
>
> Taking a quick glance at the code, it looks like this would be a good
> place to do some optimization in the BatchScanner's impl
> (TabletServerBatchReaderImpl). The BatchScanner will bin the ranges to the
> tablets and the servers hosting those tablets. Normally, this would be
> spread out, but, in your single server case, all 1M rows would all go to a
> single TabletServer in one RPC call.
>
> I'm guessing a good optimization here would be to check the size of a
> batch of Ranges for a single tabletserver, and when above a certain
> threshold, split the batch in half and try to reprocess each half (the
> recursion would naturally keep splitting until we get down to some
> high-watermark).
>
> Point being, if your client VM constructed the Ranges without issue, the
> BatchScanner impl should be smart enough to not knock over a TabletServer.
>
> What was the reason the tserver died? OOME? Was there anything at the end
> of the log files or in the .out/.err files?
>
> - Josh
>
>
> Dylan Hutchison wrote:
>
>> A friend of mine has a use case where he wants to scan ~1M individual
>> rows, scattered across a ~15GB table.  He performed the following:
>>
>> 1. Gather a List of Range objects, each one a singleton range spanning
>> an entire row.
>> 2. Create a BatchScanner with one read thread.
>> 3. Set the ranges via BatchScanner.setRanges()
>> 4. Start iterating through the scanner.
>>
>> Performing these steps crashed the TabletServer for my friend (haven't
>> had time to verify it myself yet). We're using a single-node standalone
>> 1.6.1 Accumulo instance.
>>
>> Is this a bad way to use Accumulo?  I advised my friend to batch the
>> reads into groups of ~10k ranges and see if that helps.  I wanted to
>> check with the community and see if we're doing something weird.  If the
>> behavior should have worked, I can try to put together a test case
>> reproducing it, that creates a table with many entries and then scans
>> with many ranges.
>>
>> Thanks,
>> Dylan Hutchison
>>
>>

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