On 25 Jan 2013, at 10:37, Bertrand Dechoux wrote:
It seems to me the question has not been answered :
"is it possible yes or no to force a smaller split size
than a block on the mappers"
Not that I know (but you could implement something to do it) but why
would
you do it?
By default if the split is set under the size of a block, it will be a
block.
One of the reason is data-locality. The second is that a block is
written
into a single hard-drive (leaving replicas aside) so if n mappers were
reading n parts from the same block well they would share the
hard-drive
bandwidth... So it is not a clear win.
You can change the block size of the file you want to read but using
smaller block size is really an anti-pattern. Most people increase the
block size.
(Note : block size of files are fixed when writing the files and it
can be
different between two different files.)
That will be my approach for now, or disabling compression altogether
for
these files. The only problem I have is that compression is so efficient
that any operation in the mapper (so on the uncompressed data) just
makes
the mapper throw an OOM exception, no matter how much memory I give it.
What partly works though, is setting a low mapred.max.split.size. In a
directory containing 34 files, I get 33 mappers (???). When setting
hive.merge.mapfiles to false (and leaving mapred.max.split.size at its
fs
blocksize default), it doesn't seem to have any effect and I get 20
mappers
only.
Are you trying to handle data which are too small?
If hive supports multi-threading for mapper it might be an solution.
But I
don't the configuration for that.
Regards
Bertrand
PS : the question is quite general and not really hive related
I realized that after re-reading the whole thread :-)
Thanks for all the answers, everyone!
David
On Fri, Jan 25, 2013 at 8:46 AM, Edward Capriolo
<edlinuxg...@gmail.com>wrote:
Not all files are split-table Sequence Files are. Raw gzip files are
not.
On Fri, Jan 25, 2013 at 1:47 AM, Nitin Pawar
<nitinpawar...@gmail.com>wrote:
set mapred.min.split.size=1024000;
set mapred.max.split.size=4096000;
set hive.merge.mapfiles=false;
I had set above value and setting max split size to a lower value
did
increase my # number of maps. My blocksize was 128MB
Only thing was my files on hdfs were not heavily compressed and I
was
using RCFileFormat
I would suggest if you have heavily compressed files then you may
want to
do check what will be size after uncompression and allocate more
memory to
maps
On Fri, Jan 25, 2013 at 11:46 AM, David Morel <dmore...@gmail.com>
wrote:
Hello,
I have seen many posts on various sites and MLs, but didn't find a
firm
answer anywhere: is it possible yes or no to force a smaller split
size
than a block on the mappers, from the client side? I'm not after
pointers to the docs (unless you're very very sure :-) but after
real-life experience along the lines of 'yes, it works this way,
I've
done it like this...'
All the parameters that I could find (especially specifying a max
input
split size) seem to have no effect, and the files that I have are
so
heavily compressed that they completely saturate the mappers'
memory
when processed.
A solution I could imagine for this specific issue is reducing the
block
size, but for now I simply went with disabling in-file compression
for
those. And changing the block size on a per-file basis is something
I'd
like to avoid if at all possible.
All the hive settings that we tried only got me as far as raising
the
number of mappers from 5 to 6 (yay!) where I would have needed at
least
ten times more.
Thanks!
D.Morel
--
Nitin Pawar
--
Bertrand Dechoux
