[英]Python: Using multiprocessing module as possible solution to increase the speed of my function
我用Python 2.7(在Windows OS 64位上)編寫了一個函數,以便根據ESRI shapefile格式的參考多邊形(Ref)和一個或多個分段(Seg)多邊形計算相交區域的平均值。 該代碼非常慢,因為我有2000多個參考多邊形,並且對於每個Ref_polygon,該函數每次都會對所有Seg多邊形(超過7000個)運行。 抱歉,該函數是原型。
我想知道多處理是否可以幫助我提高循環速度,或者有更多的性能解決方案。 如果多處理可能是一種解決方案,我希望知道優化我的后續功能的最佳方法
import numpy as np
import ogr
import osr,gdal
from shapely.geometry import Polygon
from shapely.geometry import Point
import osgeo.gdal
import osgeo.gdal as gdal
def AreaInter(reference,segmented,outFile):
# open shapefile
ref = osgeo.ogr.Open(reference)
if ref is None:
raise SystemExit('Unable to open %s' % reference)
seg = osgeo.ogr.Open(segmented)
if seg is None:
raise SystemExit('Unable to open %s' % segmented)
ref_layer = ref.GetLayer()
seg_layer = seg.GetLayer()
# create outfile
if not os.path.split(outFile)[0]:
file_path, file_name_ext = os.path.split(os.path.abspath(reference))
outFile_filename = os.path.splitext(os.path.basename(outFile))[0]
file_out = open(os.path.abspath("{0}\\{1}.txt".format(file_path, outFile_filename)), "w")
else:
file_path_name, file_ext = os.path.splitext(outFile)
file_out = open(os.path.abspath("{0}.txt".format(file_path_name)), "w")
# For each reference objects-i
for index in xrange(ref_layer.GetFeatureCount()):
ref_feature = ref_layer.GetFeature(index)
# get FID (=Feature ID)
FID = str(ref_feature.GetFID())
ref_geometry = ref_feature.GetGeometryRef()
pts = ref_geometry.GetGeometryRef(0)
points = []
for p in xrange(pts.GetPointCount()):
points.append((pts.GetX(p), pts.GetY(p)))
# convert in a shapely polygon
ref_polygon = Polygon(points)
# get the area
ref_Area = ref_polygon.area
# create an empty list
Area_seg, Area_intersect = ([] for _ in range(2))
# For each segmented objects-j
for segment in xrange(seg_layer.GetFeatureCount()):
seg_feature = seg_layer.GetFeature(segment)
seg_geometry = seg_feature.GetGeometryRef()
pts = seg_geometry.GetGeometryRef(0)
points = []
for p in xrange(pts.GetPointCount()):
points.append((pts.GetX(p), pts.GetY(p)))
seg_polygon = Polygon(points)
seg_Area.append = seg_polygon.area
# intersection (overlap) of reference object with the segmented object
intersect_polygon = ref_polygon.intersection(seg_polygon)
# area of intersection (= 0, No intersection)
intersect_Area.append = intersect_polygon.area
# Avarage for all segmented objects (because 1 or more segmented polygons can intersect with reference polygon)
seg_Area_average = numpy.average(seg_Area)
intersect_Area_average = numpy.average(intersect_Area)
file_out.write(" ".join(["%s" %i for i in [FID, ref_Area,seg_Area_average,intersect_Area_average]])+ "\n")
file_out.close()
您可以使用多處理程序包,尤其是Pool
類。 首先創建一個函數,該函數執行您要在for循環中完成的所有工作,並且僅將索引作為參數:
def process_reference_object(index):
ref_feature = ref_layer.GetFeature(index)
# all your code goes here
return (" ".join(["%s" %i for i in [FID, ref_Area,seg_Area_average,intersect_Area_average]])+ "\n")
請注意 ,這不會寫入文件本身,那樣會很麻煩,因為您將有多個進程同時寫入同一文件。 而是返回需要寫入的字符串。 還要注意,此函數中有些對象需要以某種方式到達它,例如ref_layer
或ref_geometry
由您決定如何執行(您可以將process_reference_object
作為方法初始化在使用它們初始化的類中,或者可能很難看懂。只是在全局范圍內進行定義)。
然后,您創建一個進程資源池,並使用Pool.imap_unordered
(它將根據需要將每個索引本身分配給不同的進程)運行所有索引:
from multiprocessing import Pool
p = Pool() # run multiple processes
for l in p.imap_unordered(process_reference_object, range(ref_layer.GetFeatureCount())):
file_out.write(l)
這將並行化多個過程中對參考對象的獨立處理,並將它們寫入文件(按任意順序,請注意)。
線程可以在一定程度上有所幫助,但是首先您應該確保不能簡化算法。 如果要對照7000個分段多邊形檢查2000個參考多邊形中的每一個(也許我誤解了),那么應該從那里開始。 運行在O(n 2 )的東西將會很慢,因此也許您可以修剪掉絕對不會相交的東西,或者找到其他加快速度的方法。 否則,運行多個進程或線程只會在數據幾何增長時線性改善。
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