问题
I am looking to parallelise numpy or pandas operations. For this I have been looking into pydata's blaze. My understanding was that seemless parallelisation was its major selling point.
Unfortunately I have been unable to find an operation that runs on more than one core. Is parallel processing in blaze available yet or currently only a stated aim? Am I doing something wrong? I am using blaze v0.6.5.
Example of one function I was hoping to parallelise: (deduplication of a pytables column too large to fit in memory)
import pandas as pd
import blaze as bz
def f1():
counter = 0
groups = pd.DataFrame(columns=['name'])
t = bz.TableSymbol('t', '{name: string}')
e = bz.distinct(t)
for chunk in store.select('my_names', columns=['name'],
chunksize=1e5):
counter += 1
print('processing chunk %d' % counter)
groups = pd.concat([groups, chunk])
groups = bz.compute(e, groups)
Edit 1
I have had problems following Phillip's examples:
In [1]: from blaze import Data, compute
In [2]: d = Data('test.bcolz')
In [3]: d.head(5)
Out[3]: <repr(<blaze.expr.collections.Head at 0x7b5e300>) failed: NotImplementedError: Don't know how to compute:
expr: _1.head(5).head(11)
data: {_1: ctable((8769257,), [('index', '<i8'), ('date', 'S10'), ('accessDate', 'S26')])
nbytes: 367.97 MB; cbytes: 35.65 MB; ratio: 10.32
cparams := cparams(clevel=5, shuffle=True, cname='blosclz')
rootdir := 'test.bcolz'
[(0L, '2014-12-12', '2014-12-14T17:39:19.716000')
(1L, '2014-12-11', '2014-12-14T17:39:19.716000')
(2L, '2014-12-10', '2014-12-14T17:39:19.716000') ...,
(1767L, '2009-11-11', '2014-12-15T13:32:39.906000')
(1768L, '2009-11-10', '2014-12-15T13:32:39.906000')
(1769L, '2009-11-09', '2014-12-15T13:32:39.906000')]}>
My environment:
C:\Anaconda>conda list blaze
# packages in environment at C:\Anaconda:
#
blaze 0.6.8 np19py27_69
But note, blaze seems to report a wrong version:
In [5]: import blaze
In [6]: blaze.__version__
Out[6]: '0.6.7'
With other data sources blaze seems to work:
In [6]: d = Data([1,2,2,2,3,4,4,4,5,6])
In [7]: d.head(5)
Out[7]:
_2
0 1
1 2
2 2
3 2
4 3
In [16]: list(compute(d._2.distinct()))
Out[16]: [1, 2, 3, 4, 5, 6]
回答1:
Note: The example below requires the latest version of blaze, which you can get via
conda install -c blaze blaze
You'll also need the latest version of the nascent into project. You'll need to install into from master, which you can do with
pip install git+git://github.com/ContinuumIO/into.git
You can't do "seamless" parallelization with an arbitrary backend, but the bcolz backend supports parallelization in a nice way. Here's an example with the NYC Taxi trip/fare dataset
Note: I've combined both the trip and fare datasets into a single dataset. There are 173,179,759 rows in the dataset
In [28]: from blaze import Data, compute
In [29]: ls -d *.bcolz
all.bcolz/ fare.bcolz/ trip.bcolz/
In [30]: d = Data('all.bcolz')
In [31]: d.head(5)
Out[31]:
medallion hack_license \
0 89D227B655E5C82AECF13C3F540D4CF4 BA96DE419E711691B9445D6A6307C170
1 0BD7C8F5BA12B88E0B67BED28BEA73D8 9FD8F69F0804BDB5549F40E9DA1BE472
2 0BD7C8F5BA12B88E0B67BED28BEA73D8 9FD8F69F0804BDB5549F40E9DA1BE472
3 DFD2202EE08F7A8DC9A57B02ACB81FE2 51EE87E3205C985EF8431D850C786310
4 DFD2202EE08F7A8DC9A57B02ACB81FE2 51EE87E3205C985EF8431D850C786310
vendor_id rate_code store_and_fwd_flag pickup_datetime \
0 CMT 1 N 2013-01-01 15:11:48
1 CMT 1 N 2013-01-06 00:18:35
2 CMT 1 N 2013-01-05 18:49:41
3 CMT 1 N 2013-01-07 23:54:15
4 CMT 1 N 2013-01-07 23:25:03
dropoff_datetime passenger_count trip_time_in_secs trip_distance \
0 2013-01-01 15:18:10 4 382 1.0
1 2013-01-06 00:22:54 1 259 1.5
2 2013-01-05 18:54:23 1 282 1.1
3 2013-01-07 23:58:20 2 244 0.7
4 2013-01-07 23:34:24 1 560 2.1
... pickup_latitude dropoff_longitude dropoff_latitude \
0 ... 40.757977 -73.989838 40.751171
1 ... 40.731781 -73.994499 40.750660
2 ... 40.737770 -74.009834 40.726002
3 ... 40.759945 -73.984734 40.759388
4 ... 40.748528 -74.002586 40.747868
tolls_amount tip_amount total_amount mta_tax fare_amount payment_type \
0 0 0 7.0 0.5 6.5 CSH
1 0 0 7.0 0.5 6.0 CSH
2 0 0 7.0 0.5 5.5 CSH
3 0 0 6.0 0.5 5.0 CSH
4 0 0 10.5 0.5 9.5 CSH
surcharge
0 0.0
1 0.5
2 1.0
3 0.5
4 0.5
[5 rows x 21 columns]
To add process-based parallelism, we bring in the Pool class from the multiprocessing stdlib module, and pass the Pool instance's map method as a keyword argument to compute:
In [32]: from multiprocessing import Pool
In [33]: p = Pool()
In [34]: %timeit -n 1 -r 1 values = compute(trip.medallion.distinct())
1 loops, best of 1: 1min per loop
In [35]: %timeit -n 1 -r 1 values = compute(trip.medallion.distinct(), map=p.map)
1 loops, best of 1: 16.2 s per loop
So, roughly a 3x speedup for an extra line of code. Note that this is a string column, and these tend to be very inefficient compared to other types. An distinct expression computed over an integer column is finished in about 1 second (vs 3 seconds) with multiple cores (so, about the same improvement in running time):
In [38]: %timeit -n 1 -r 1 values = compute(trip.passenger_count.distinct())
1 loops, best of 1: 3.33 s per loop
In [39]: %timeit -n 1 -r 1 values = compute(trip.passenger_count.distinct(), map=p.map)
1 loops, best of 1: 1.01 s per loop
来源：https://stackoverflow.com/questions/27505764/pydata-blaze-does-it-allow-parallel-processing-or-not