It is because of the nested distinct operation= relies on the RAM to calculate unique values.

As described here: http://stackoverflow.com= /questions/10732456/how-to-optimize-a-group-by-statement-in-pig-latin<= span arial,=3D"" '";=3D"" font-size:=3D"" 14px;=3D"" color:=3D"" rgb(0,=3D= "" 0,=3D"" 0);=3D"" background-color:=3D"" rgb(255,=3D"" 255,=3D"" 255);= =3D"" font-weight:=3D"" normal;=3D"" font-style:=3D"" normal;text-decorati= on:=3D"" none;'=3D"" style=3D"font-family: ''; font-size: 10.5pt; line-hei= ght: 1.5; background-color: window;">

=0A

Thanks,Lei

=0A

leiwangouc@gmail.com

<= /div>=0A

From:&n= bsp;leiwangouc@gmail.com
=
Date: 2014-04-16 11:58
To: user; th; german.fl; user
Subject:&nb= sp;Re: Re: java.lang.OutOfMemoryError related with number of reducer?
=0A
= Hi German & Thomas,

Seem= s i found the data that causes the error, but i still don't know the exact= ly reason.

I just do a group with p= ig latin:

domain_device_group =3D GROUP data_filte= r BY (custid, domain, level, device); =0A
domain_device =3D FOREACH dom= ain_device_group {=0A
distinct_ip =3D DISTINCT data_filter.ip;=0A distinct_userid =3D DISTINCT data_filter.use= rid;=0A
GENERATE group.custid, group.domain= , group.level, group.device, COUNT_STAR(data_filter), COUNT_STAR(distinct_= ip), COUNT_STAR(distinct_userid);=0A
}=0A
STORE domain_device INTO '= $outputdir/$batchdate/data/domain_device' USING PigStorage('\t');=0A

The group key (custid, domain, level, devi= ce) is significantly skewed, about 42% () of the records are the same key, a= nd only the reducer which handle this key failed.
But fro= m https://www.inkling.com/read/hadoop-definitive-guide-tom-white-3rd/ch= apter-6/shuffle-and-sort , I still have no idea why it caus= e an OOM. It doesn't tell how skewed key will be handled, neither ho= w different keys in same reducer will be merged.
=0A
leiwangouc@gmail.com
=0A

From: leiwangouc@gmail.com
Dat= e: 2014-04-15 23:35
To: user; th<= /a>; german.fl
S= ubject: Re: RE: memoryjava.lang.OutOfMemoryError related with num= ber of reducer?
=0A=0A
Thanks,= let me take a careful look at it.
=0A

=0A
leiwangouc@gmail.com
=0A
&n= bsp;
From: German Florez-Larrahondo
Date: 2014= -04-15 23:27
To: user; <= a href=3D"mailto:th@bentzn.com" style=3D"color: blue; text-decoration: und= erline;">'th'
Subject: RE: Re: memoryjava.lang.O= utOfMemoryError related with number of reducer?