Hi Andrew,
On Thu, Jun 7, 2012 at 5:39 PM, Andrew Myers
Hi Cafe, I'm working on inspecting some data that I'm trying to represent as records in Haskell and seeing about twice the memory footprint than I was expecting. I've got roughly 1.4 million records in a CSV file (400M on disk) that I parse in using bytestring-csv. bytestring-csv returns a [[ByteString]] (wrapped in `type`s) which I then convert into a list of records that have the following structure:
3 Int 1 Text Length 3 1 Text Length 11 12 Float 1 UTCTime
All fields are marked strict and have {-# UNPACK #-} pragmas (I'm guessing that doesn't do anything for non primitives). (Side note, is there a way to check if things are actually being unpacked?)
GHC used to complain when you use UNPACK with something that can't be unpacked, but that warning seems to have been (accidentally) removed in 7.4.1. The rule for unpacking is: * all product types (i.e. types with only one constructor) can be unpacked. This includes Int, Char, Double, etc and tuples or records their-of. * sum types (i.e. data types with more than one constructor) and polymorphic fields can't be unpacked.
My back of the napkin memory estimates based on the assumption that nothing is being unpacked (and my very spotty understanding of Haskell data structures):
Platform: 64 Bit Linux # Type (Sizeof type (occasionally a guess))
3 * Int (8) 14 * Char (4) -- Text is some kind of bytestring but I'm guessing it can't be worse than the same number of Char? 12 * Float (4) 18 * sizeOf (ptr) (8) UTC: -- From what I can gather through :info in ghci 4 * (ptr) (8) 2 * Integer (16) -- Shouldn't be overly large, times are within 2012
All fields in a constructor are word aligned. This means that all primitive types take 8 bytes on a 64-bit platform, including Char and Float. You might find the following blog posts by me useful in computing the size of data structures: http://blog.johantibell.com/2011/06/memory-footprints-of-some-common-data.ht... http://blog.johantibell.com/2011/06/computing-size-of-hashmap.html http://blog.johantibell.com/2011/11/slides-from-my-guest-lecture-at.html Here's some more on the topic: http://stackoverflow.com/questions/3254758/memory-footprint-of-haskell-data-... http://stackoverflow.com/questions/6574444/how-to-find-out-ghcs-memory-repre...
I've written a small driver test program that just parses the CSV, finds the minimum value for a couple of the Float fields, and exits. In the process monitor the memory usage is 6.9G before the program exits. I've tried profiling with +RTS -hc but it ran for >3 hours without finishing, it normally finishes within 4 minutes. Anyone have any ideas for me? Things to try? Thanks, Andrew
You could try to use a 32-bit GHC, which would use about half the memory. You're at the limit of the size of data that you can comfortably fit in memory on a normal desktop machine, so it might be time to consider a streaming approach. -- Johan