Alan Gates wrote:
On May 19, 2009, at 10:30 PM, Mridul Muralidharan wrote:
I am still not very convinced about the value about this
implementation - particularly considering the advances made since 1.3
in memory allocators and garbage collection.
My fundamental concern is not with the slowness of garbage collection.
I am asserting (along with the paper) that garbage collection is not an
optimal choice for a large data processing system. I don't want to
improve the garbage collector, I want to manage a subset of the memory
without it.
I should probably have elaborated better.
Most objects in Pig are in young generation (pls correct me if I am
wrong) - so promoting them from there (which is handled pretty optimally
and blazingly fast by vm) into slower/longer memory pools should be done
with some thought (management of buffers, etc).
The only (corner) cases where this is not valid, from top of my head, is
when a single tuple becomes really large due to a bag (usually) with
either large number of tuples in it, or tuples with larger payloads :
and imo that results in quite similar costs with this proposal too - but
I could be wrong.
The side effect of this proposal is many, and sometimes non-obvious.
Like implicitly moving young generation data into older generation,
causing much more memory pressure for gc, fragmentation of memory
blocks causing quite a bit of memory pressure, replicating quite a bit
of functionality with garbage collection, possibility of bugs with ref
counting, etc.
I don't understand your concerns regarding the load on the gc and memory
fragmentation. Let's say I have 10,000 tuples, each with 10 fields.
Let's also assume that these tuples live long enough to make it into the
"old" memory pool, since this is the interesting case where objects live
long enough to cause a problem. In the current implementation there
will be 110,000 objects that the gc has to manage moving into the old
pool, and check every time it cleans the old pool. In the proposed
implementation there would be 10,001 objects (assuming all the data fit
into one buffer) to manage. And rather than allocating 100,000 small
pieces of memory, we would have allocated one large segment. My belief
is that this would lighten the load on the gc.
Old gen memory management is not very trivial.
For example, which should probably be very commonly known now - if an
old block is freed and yet the cost of moving existing blocks around to
use the 'free' block is high, vm just leaves it around. Over time, you
will end up with fragmentation on old gen which cant be freed. (This is
not a vm bug - the costs outweigh the benefits).
That being said, as I mentioned above, the costs of mem usage is not
linear - young gen is way faster (allocation, management, free) than
objects promoted to older generations (successively) [compaction,
reference changes, etc in gc].
In pig's case, since it is essentially streaming in nature - most
tuples/bag - except in corner cases, would fall into young gen where
things are faster.
Just a note though -
The last time I had to dabble in memory management for my server needs,
it was already pretty complex and un-intutive (not to mention env and
impl specific) - and that was a few years back - unfortunately, I have
not kept abreast with recent changes (and quite a few have gone into vm
for java 6 I was told) : so probably my comments above might not be
valid anymore.
Other than saying you would probably want to test extensively like we
had to do, and that things are not as simple as they normally appear
[and imo almost all books/articles get it wrong - so testing is only way
out], I cant really comment more authoritatively anymore :-) Any
improvement to pig memory management would be a welcome change though !
Regards,
Mridul
This does replicate some of the functionality of the garbage
collector. Complex systems frequently need to re-implement foundational
functionality in order to optimize it for their needs. Hence many RDBMS
engines have their own implementations of memory management, file I/O,
thread scheduling, etc.
As for bugs in ref counting, I agree that forgetting to deallocate is
one of the most pernicious problems of allowing programmers to do memory
management. But in this case all that will happen is that a buffer will
get left around that isn't needed. If the system needs more memory then
that buffer will eventually get selected for flushing to disk, and then
it will stay there as no one will call it back into memory. So the cost
of forgetting to deallocate is minor.
If assumption that current working set of bag/tuple does not need to
be spilled, and anything else can be, then this will pretty much
deteriorate to current impl in worst case.
That is not the assumption. There are two issues: 1) trying to spill
bags only when we determine we need to is highly error prone, because we
can't accurately determine when we need to and because we sometimes
can't dump fast enough to survive; 2) current memory usage is far too
high, and needs to be reduced.
A much more simpler method to gain benefits would be to handle
primitives as ... primitives and not through the java wrapper classes
for them.
It should be possible to write schema aware tuples which make use of
the primitives specified to take a fraction of memory required (4
bytes + null_check boolean for int + offset mapping instead of 24/32
bytes it currently is, etc).
In my observation, at least 50% of the data in pig is untyped, which
means it's a byte array. Of the 50% that people declare or is
determined by the program, probably 50-80% of that are chararrays and
maps. So that means that somewhere around <25% of the data is numeric.
Shrinking that 25% by 75% will be nice, but not adequate. And it does
nothing to help with the issue of being able to spill in a controlled
way instead of only in emergency situations.
Alan.
