使用 boto3 时垃圾收集不正常

Question

I'm just running the code to listen to sqs messages in a loop for n times. After each iteration I'm calling gc.collect() but it's returning some unreachable objects and also I'm checking the gc.garbagge for the objects not collected by gc this list also keep on increasing with each iteration.

Sample Code:

import os
import gc
import boto3
import psutil

gc.set_debug(gc.DEBUG_SAVEALL | gc.DEBUG_UNCOLLECTABLE)
def get_memory_usage():
    return psutil.Process(os.getpid()).memory_info().rss // 1024 ** 2


def test():
    queue_url = 'https://sqs.us-east-2.amazonaws.com/123/test.fifo'
    sqs = create_client('sqs')
    for i in range(250):
        message = sqs.receive_message(QueueUrl=queue_url)
        if message.get('Messages'):
            recept_handle = message['Messages'][0]['ReceiptHandle']
            sqs.delete_message(QueueUrl=queue_url, ReceiptHandle=recept_handle)

        print(f'Iteration - {i + 1} Unreachable Objects: {gc.collect()} and length: {len(gc.garbage)}')


print(f'Memory usage Before: {get_memory_usage()}mb')
test()
print(f'==================Unreachable Objects: {gc.collect()}==================')
print(len(gc.garbage))
print(f'Memory usage After: {get_memory_usage()}mb')

Output:

Memory usage Before: 27mb
Iteration - 1 Unreachable Objects: 449 and length: 554
Iteration - 2 Unreachable Objects: 3 and length: 557
Iteration - 3 Unreachable Objects: 3 and length: 560
Iteration - 4 Unreachable Objects: 3 and length: 563
Iteration - 5 Unreachable Objects: 3 and length: 566
Iteration - 6 Unreachable Objects: 3 and length: 569
Iteration - 7 Unreachable Objects: 3 and length: 572
Iteration - 8 Unreachable Objects: 3 and length: 575
Iteration - 9 Unreachable Objects: 3 and length: 578
Iteration - 10 Unreachable Objects: 3 and length: 581
Iteration - 11 Unreachable Objects: 3 and length: 584
Iteration - 12 Unreachable Objects: 3 and length: 587
Iteration - 13 Unreachable Objects: 3 and length: 590
Iteration - 14 Unreachable Objects: 3 and length: 593
Iteration - 15 Unreachable Objects: 3 and length: 596
Iteration - 16 Unreachable Objects: 3 and length: 599
Iteration - 17 Unreachable Objects: 3 and length: 602
Iteration - 18 Unreachable Objects: 3 and length: 605
Iteration - 19 Unreachable Objects: 3 and length: 608
Iteration - 20 Unreachable Objects: 3 and length: 611
Iteration - 21 Unreachable Objects: 3 and length: 614
Iteration - 22 Unreachable Objects: 3 and length: 617
Iteration - 23 Unreachable Objects: 3 and length: 620
Iteration - 24 Unreachable Objects: 3 and length: 623
Iteration - 25 Unreachable Objects: 3 and length: 626
Iteration - 26 Unreachable Objects: 3 and length: 629
Iteration - 27 Unreachable Objects: 3 and length: 632
Iteration - 28 Unreachable Objects: 3 and length: 635
Iteration - 29 Unreachable Objects: 3 and length: 638
Iteration - 30 Unreachable Objects: 3 and length: 641
Iteration - 31 Unreachable Objects: 3 and length: 644
Iteration - 32 Unreachable Objects: 3 and length: 647
Iteration - 33 Unreachable Objects: 3 and length: 650
Iteration - 34 Unreachable Objects: 3 and length: 653
Iteration - 35 Unreachable Objects: 3 and length: 656
Iteration - 36 Unreachable Objects: 3 and length: 659
Iteration - 37 Unreachable Objects: 3 and length: 662
Iteration - 38 Unreachable Objects: 3 and length: 665
Iteration - 39 Unreachable Objects: 3 and length: 668
Iteration - 40 Unreachable Objects: 3 and length: 671
Iteration - 41 Unreachable Objects: 3 and length: 674
Iteration - 42 Unreachable Objects: 3 and length: 677
Iteration - 43 Unreachable Objects: 3 and length: 680
Iteration - 44 Unreachable Objects: 3 and length: 683
Iteration - 45 Unreachable Objects: 3 and length: 686
Iteration - 46 Unreachable Objects: 3 and length: 689
Iteration - 47 Unreachable Objects: 3 and length: 692
Iteration - 48 Unreachable Objects: 3 and length: 695
Iteration - 49 Unreachable Objects: 3 and length: 698
Iteration - 50 Unreachable Objects: 3 and length: 701
Iteration - 51 Unreachable Objects: 3 and length: 704
Iteration - 52 Unreachable Objects: 3 and length: 707
Iteration - 53 Unreachable Objects: 3 and length: 710
Iteration - 54 Unreachable Objects: 3 and length: 713
Iteration - 55 Unreachable Objects: 3 and length: 716
Iteration - 56 Unreachable Objects: 3 and length: 719
Iteration - 57 Unreachable Objects: 3 and length: 722
Iteration - 58 Unreachable Objects: 3 and length: 725
Iteration - 59 Unreachable Objects: 3 and length: 728
Iteration - 60 Unreachable Objects: 3 and length: 731
Iteration - 61 Unreachable Objects: 3 and length: 734
Iteration - 62 Unreachable Objects: 3 and length: 737
Iteration - 63 Unreachable Objects: 3 and length: 740
Iteration - 64 Unreachable Objects: 3 and length: 743
Iteration - 65 Unreachable Objects: 3 and length: 746
Iteration - 66 Unreachable Objects: 3 and length: 749
Iteration - 67 Unreachable Objects: 3 and length: 752
Iteration - 68 Unreachable Objects: 3 and length: 755
Iteration - 69 Unreachable Objects: 3 and length: 758
Iteration - 70 Unreachable Objects: 3 and length: 761
Iteration - 71 Unreachable Objects: 3 and length: 764
Iteration - 72 Unreachable Objects: 3 and length: 767
Iteration - 73 Unreachable Objects: 3 and length: 770
Iteration - 74 Unreachable Objects: 3 and length: 773
Iteration - 75 Unreachable Objects: 3 and length: 776
Iteration - 76 Unreachable Objects: 3 and length: 779
Iteration - 77 Unreachable Objects: 3 and length: 782
Iteration - 78 Unreachable Objects: 3 and length: 785
Iteration - 79 Unreachable Objects: 3 and length: 788
Iteration - 80 Unreachable Objects: 3 and length: 791
Iteration - 81 Unreachable Objects: 3 and length: 794
Iteration - 82 Unreachable Objects: 3 and length: 797
Iteration - 83 Unreachable Objects: 3 and length: 800
Iteration - 84 Unreachable Objects: 3 and length: 803
Iteration - 85 Unreachable Objects: 3 and length: 806
Iteration - 86 Unreachable Objects: 3 and length: 809
Iteration - 87 Unreachable Objects: 3 and length: 812
Iteration - 88 Unreachable Objects: 3 and length: 815
Iteration - 89 Unreachable Objects: 3 and length: 818
Iteration - 90 Unreachable Objects: 3 and length: 821
Iteration - 91 Unreachable Objects: 3 and length: 824
Iteration - 92 Unreachable Objects: 3 and length: 827
Iteration - 93 Unreachable Objects: 3 and length: 830
Iteration - 94 Unreachable Objects: 3 and length: 833
Iteration - 95 Unreachable Objects: 3 and length: 836
Iteration - 96 Unreachable Objects: 3 and length: 839
Iteration - 97 Unreachable Objects: 3 and length: 842
Iteration - 98 Unreachable Objects: 3 and length: 845
Iteration - 99 Unreachable Objects: 3 and length: 848
Iteration - 100 Unreachable Objects: 3 and length: 851
Iteration - 101 Unreachable Objects: 3 and length: 854
Iteration - 102 Unreachable Objects: 3 and length: 857
Iteration - 103 Unreachable Objects: 3 and length: 860
Iteration - 104 Unreachable Objects: 3 and length: 863
Iteration - 105 Unreachable Objects: 3 and length: 866
Iteration - 106 Unreachable Objects: 3 and length: 869
Iteration - 107 Unreachable Objects: 3 and length: 872
Iteration - 108 Unreachable Objects: 3 and length: 875
Iteration - 109 Unreachable Objects: 3 and length: 878
Iteration - 110 Unreachable Objects: 3 and length: 881
Iteration - 111 Unreachable Objects: 3 and length: 884
Iteration - 112 Unreachable Objects: 3 and length: 887
Iteration - 113 Unreachable Objects: 3 and length: 890
Iteration - 114 Unreachable Objects: 3 and length: 893
Iteration - 115 Unreachable Objects: 3 and length: 896
Iteration - 116 Unreachable Objects: 3 and length: 899
Iteration - 117 Unreachable Objects: 3 and length: 902
Iteration - 118 Unreachable Objects: 3 and length: 905
Iteration - 119 Unreachable Objects: 3 and length: 908
Iteration - 120 Unreachable Objects: 3 and length: 911
Iteration - 121 Unreachable Objects: 3 and length: 914
Iteration - 122 Unreachable Objects: 3 and length: 917
Iteration - 123 Unreachable Objects: 3 and length: 920
Iteration - 124 Unreachable Objects: 3 and length: 923
Iteration - 125 Unreachable Objects: 3 and length: 926
Iteration - 126 Unreachable Objects: 3 and length: 929
Iteration - 127 Unreachable Objects: 3 and length: 932
Iteration - 128 Unreachable Objects: 3 and length: 935
Iteration - 129 Unreachable Objects: 3 and length: 938
Iteration - 130 Unreachable Objects: 3 and length: 941
Iteration - 131 Unreachable Objects: 3 and length: 944
Iteration - 132 Unreachable Objects: 3 and length: 947
Iteration - 133 Unreachable Objects: 3 and length: 950
Iteration - 134 Unreachable Objects: 3 and length: 953
Iteration - 135 Unreachable Objects: 3 and length: 956
Iteration - 136 Unreachable Objects: 3 and length: 959
Iteration - 137 Unreachable Objects: 3 and length: 962
Iteration - 138 Unreachable Objects: 3 and length: 965
Iteration - 139 Unreachable Objects: 3 and length: 968
Iteration - 140 Unreachable Objects: 3 and length: 971
Iteration - 141 Unreachable Objects: 3 and length: 974
Iteration - 142 Unreachable Objects: 3 and length: 977
Iteration - 143 Unreachable Objects: 3 and length: 980
Iteration - 144 Unreachable Objects: 3 and length: 983
Iteration - 145 Unreachable Objects: 3 and length: 986
Iteration - 146 Unreachable Objects: 3 and length: 989
Iteration - 147 Unreachable Objects: 3 and length: 992
Iteration - 148 Unreachable Objects: 3 and length: 995
Iteration - 149 Unreachable Objects: 3 and length: 998
Iteration - 150 Unreachable Objects: 3 and length: 1001
Iteration - 151 Unreachable Objects: 3 and length: 1004
Iteration - 152 Unreachable Objects: 3 and length: 1007
Iteration - 153 Unreachable Objects: 3 and length: 1010
Iteration - 154 Unreachable Objects: 3 and length: 1013
Iteration - 155 Unreachable Objects: 3 and length: 1016
Iteration - 156 Unreachable Objects: 3 and length: 1019
Iteration - 157 Unreachable Objects: 3 and length: 1022
Iteration - 158 Unreachable Objects: 3 and length: 1025
Iteration - 159 Unreachable Objects: 3 and length: 1028
Iteration - 160 Unreachable Objects: 3 and length: 1031
Iteration - 161 Unreachable Objects: 3 and length: 1034
Iteration - 162 Unreachable Objects: 3 and length: 1037
Iteration - 163 Unreachable Objects: 3 and length: 1040
Iteration - 164 Unreachable Objects: 3 and length: 1043
Iteration - 165 Unreachable Objects: 3 and length: 1046
Iteration - 166 Unreachable Objects: 3 and length: 1049
Iteration - 167 Unreachable Objects: 3 and length: 1052
Iteration - 168 Unreachable Objects: 3 and length: 1055
Iteration - 169 Unreachable Objects: 3 and length: 1058
Iteration - 170 Unreachable Objects: 3 and length: 1061
Iteration - 171 Unreachable Objects: 3 and length: 1064
Iteration - 172 Unreachable Objects: 3 and length: 1067
Iteration - 173 Unreachable Objects: 3 and length: 1070
Iteration - 174 Unreachable Objects: 3 and length: 1073
Iteration - 175 Unreachable Objects: 3 and length: 1076
Iteration - 176 Unreachable Objects: 3 and length: 1079
Iteration - 177 Unreachable Objects: 3 and length: 1082
Iteration - 178 Unreachable Objects: 3 and length: 1085
Iteration - 179 Unreachable Objects: 3 and length: 1088
Iteration - 180 Unreachable Objects: 3 and length: 1091
Iteration - 181 Unreachable Objects: 3 and length: 1094
Iteration - 182 Unreachable Objects: 3 and length: 1097
Iteration - 183 Unreachable Objects: 3 and length: 1100
Iteration - 184 Unreachable Objects: 3 and length: 1103
Iteration - 185 Unreachable Objects: 3 and length: 1106
Iteration - 186 Unreachable Objects: 3 and length: 1109
Iteration - 187 Unreachable Objects: 3 and length: 1112
Iteration - 188 Unreachable Objects: 3 and length: 1115
Iteration - 189 Unreachable Objects: 3 and length: 1118
Iteration - 190 Unreachable Objects: 3 and length: 1121
Iteration - 191 Unreachable Objects: 3 and length: 1124
Iteration - 192 Unreachable Objects: 3 and length: 1127
Iteration - 193 Unreachable Objects: 3 and length: 1130
Iteration - 194 Unreachable Objects: 3 and length: 1133
Iteration - 195 Unreachable Objects: 3 and length: 1136
Iteration - 196 Unreachable Objects: 3 and length: 1139
Iteration - 197 Unreachable Objects: 3 and length: 1142
Iteration - 198 Unreachable Objects: 3 and length: 1145
Iteration - 199 Unreachable Objects: 3 and length: 1148
Iteration - 200 Unreachable Objects: 3 and length: 1151
Iteration - 201 Unreachable Objects: 3 and length: 1154
Iteration - 202 Unreachable Objects: 3 and length: 1157
Iteration - 203 Unreachable Objects: 3 and length: 1160
Iteration - 204 Unreachable Objects: 3 and length: 1163
Iteration - 205 Unreachable Objects: 3 and length: 1166
Iteration - 206 Unreachable Objects: 3 and length: 1169
Iteration - 207 Unreachable Objects: 3 and length: 1172
Iteration - 208 Unreachable Objects: 3 and length: 1175
Iteration - 209 Unreachable Objects: 3 and length: 1178
Iteration - 210 Unreachable Objects: 3 and length: 1181
Iteration - 211 Unreachable Objects: 3 and length: 1184
Iteration - 212 Unreachable Objects: 3 and length: 1187
Iteration - 213 Unreachable Objects: 3 and length: 1190
Iteration - 214 Unreachable Objects: 3 and length: 1193
Iteration - 215 Unreachable Objects: 3 and length: 1196
Iteration - 216 Unreachable Objects: 3 and length: 1199
Iteration - 217 Unreachable Objects: 3 and length: 1202
Iteration - 218 Unreachable Objects: 3 and length: 1205
Iteration - 219 Unreachable Objects: 3 and length: 1208
Iteration - 220 Unreachable Objects: 3 and length: 1211
Iteration - 221 Unreachable Objects: 3 and length: 1214
Iteration - 222 Unreachable Objects: 3 and length: 1217
Iteration - 223 Unreachable Objects: 3 and length: 1220
Iteration - 224 Unreachable Objects: 3 and length: 1223
Iteration - 225 Unreachable Objects: 3 and length: 1226
Iteration - 226 Unreachable Objects: 3 and length: 1229
Iteration - 227 Unreachable Objects: 3 and length: 1232
Iteration - 228 Unreachable Objects: 3 and length: 1235
Iteration - 229 Unreachable Objects: 3 and length: 1238
Iteration - 230 Unreachable Objects: 3 and length: 1241
Iteration - 231 Unreachable Objects: 3 and length: 1244
Iteration - 232 Unreachable Objects: 3 and length: 1247
Iteration - 233 Unreachable Objects: 3 and length: 1250
Iteration - 234 Unreachable Objects: 3 and length: 1253
Iteration - 235 Unreachable Objects: 3 and length: 1256
Iteration - 236 Unreachable Objects: 3 and length: 1259
Iteration - 237 Unreachable Objects: 3 and length: 1262
Iteration - 238 Unreachable Objects: 3 and length: 1265
Iteration - 239 Unreachable Objects: 3 and length: 1268
Iteration - 240 Unreachable Objects: 3 and length: 1271
Iteration - 241 Unreachable Objects: 3 and length: 1274
Iteration - 242 Unreachable Objects: 3 and length: 1277
Iteration - 243 Unreachable Objects: 3 and length: 1280
Iteration - 244 Unreachable Objects: 3 and length: 1283
Iteration - 245 Unreachable Objects: 3 and length: 1286
Iteration - 246 Unreachable Objects: 3 and length: 1289
Iteration - 247 Unreachable Objects: 3 and length: 1292
Iteration - 248 Unreachable Objects: 3 and length: 1295
Iteration - 249 Unreachable Objects: 3 and length: 1298
Iteration - 250 Unreachable Objects: 3 and length: 1301
==================Unreachable Objects: 233==================
1534
Memory usage After: 37mb

This is just a sample code, in my actual use case I'm running the code in infinite loop in a separate thread so memory is kept on accumulating.

Please help me in understanding the issue here is it normal or is something wrong with my code

Answer 1

Preface:

There seem to be two different questions here - the first is regarding the use of boto3 in threads ( "in my actual use case I'm running the code in infinite loop in a separate thread" ), and the other is asking about issues with the sample code provided.

Addressing the first; reportedly, boto3's Session() objects might "not play nicely" with threads in some implementation patterns:

• Clients are never GC'd #805
• Excessive memory usage on multithreading #1670

As the sample code does not demonstrate threading at all, it is hard to tell if this is the case. For this reason, I'll address the "second" question: "is something %s" with the sample code provided.

If this does not answer the question, providing a sample code which demonstrates the issue with threads would be extremely helpful.

---

If gc.collect() returns a positive number, that doesn't necessarily indicate a problem. It just means that objects with positive refcounts were collected, as none of the references to them were reachable.
As for the increase in memory usage, this is due to setting gc.DEBUG_SAVEALL . This setting will prevent actual object release as they are added to the gc.garbage list instead, and the memory used by them cannot be freed.

To demonstrate, the following code is loosely based on the sample provided:

import gc
import os
from contextlib import contextmanager
import psutil

ITERATIONS = 10
SIZE = 1024 * 1024     # 1MB

gc.set_debug(gc.DEBUG_SAVEALL | gc.DEBUG_UNCOLLECTABLE)

def get_memory_usage():
    return psutil.Process(os.getpid()).memory_info().rss // 1024 ** 2

@contextmanager
def print_ps_mem_usage(subtext):
    print(f"{'->' * 2} Memory before '{subtext}': {get_memory_usage()}mb {'->' * 2}")
    try:
        yield
    finally:
        print(f"  <- Memory after  '{subtext}': {get_memory_usage()}mb <-")

class MemContainer:
    def __init__(self, hogger, size):
        # Create a reference to MemHogger instance, facilitating a reference loop
        self.hogger = hogger
        self.megabyte = (f"{self.hogger.index} " * size)[:size]

    def __str__(self):
        return f"{self.__class__.__name__}({len(self.megabyte)/SIZE}mb)->{self.hogger!s}"

class MemHogger:
    def __init__(self, index, size):
        self.index = index
        # By keeping a link to the newly created MemContainer, we are creating a reference loop
        self.container = MemContainer(self, size)

    def __str__(self):
        return f"{self.__class__.__name__}({self.index})"

def loop_hogger():
    for i in range(ITERATIONS):
        with print_ps_mem_usage(f"Creating MemHogger {i}"):
            mh = MemHogger(i, SIZE)
        unreachable = gc.collect()
        unreachable_types = [type(unreach) for unreach in gc.garbage[-unreachable:]]
        unreachable_contents = [list(unreach.keys()) if isinstance(unreach, dict) else str(unreach) for unreach in gc.garbage[-unreachable:]]
        print(f'Iteration - {i + 1} Unreachable Objects: {unreachable} and length: {len(gc.garbage)}, items: {list(zip(unreachable_types, unreachable_contents))}')


with print_ps_mem_usage("main run"):
    loop_hogger()
    print(f'==================Unreachable Objects: {gc.collect()}==================')
    print(len(gc.garbage))

The general flow:

• MemHogger stores an index (current iteration), and a reference to a MemContainer instance. The MemContainer instance stores a ~1MB string relating to the index, and a reference to the calling MemHogger instance. This creates a reference loop between the two, which costs about 1MB of RAM.
• The first iteration declares mh as MemHogger(0) , which is still reachable at the end of that iteration; 0 objects collected.
• Following iterations reset mh to the current i , so the instance created in the previous iteration is no longer reachable from running code.
• The unreachable MemHogger instance (from the previous iteration) is still referenced by its MemContainer instance (refcount>0), so it is not immediately collected.
• At the end of the second iteration forth, 4 unreachable items are collected (from the previous iteration): MemHogger, MemHogger's __dict__ , MemContainer, and MemContainer's __dict__ . They can be reached from one-another (reference-loop), but can no longer be reached by running code.
• In the last iteration mh is set to MemHogger(9) , but mh is no longer reachable once the function returns, so the upper scope collects another 4 unreachable items.

The output should look something like this:

->-> Memory before 'main run': 10mb ->->
->-> Memory before 'Creating MemHogger 0': 10mb ->->
  <- Memory after  'Creating MemHogger 0': 11mb <-
Iteration - 1 Unreachable Objects: 0 and length: 0, items: []
->-> Memory before 'Creating MemHogger 1': 11mb ->->
  <- Memory after  'Creating MemHogger 1': 14mb <-
Iteration - 2 Unreachable Objects: 4 and length: 4, items: [(<class '__main__.MemHogger'>, 'MemHogger(0)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(0)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 2': 14mb ->->
  <- Memory after  'Creating MemHogger 2': 16mb <-
Iteration - 3 Unreachable Objects: 4 and length: 8, items: [(<class '__main__.MemHogger'>, 'MemHogger(1)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(1)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 3': 16mb ->->
  <- Memory after  'Creating MemHogger 3': 17mb <-
Iteration - 4 Unreachable Objects: 4 and length: 12, items: [(<class '__main__.MemHogger'>, 'MemHogger(2)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(2)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 4': 17mb ->->
  <- Memory after  'Creating MemHogger 4': 18mb <-
Iteration - 5 Unreachable Objects: 4 and length: 16, items: [(<class '__main__.MemHogger'>, 'MemHogger(3)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(3)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 5': 18mb ->->
  <- Memory after  'Creating MemHogger 5': 19mb <-
Iteration - 6 Unreachable Objects: 4 and length: 20, items: [(<class '__main__.MemHogger'>, 'MemHogger(4)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(4)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 6': 19mb ->->
  <- Memory after  'Creating MemHogger 6': 20mb <-
Iteration - 7 Unreachable Objects: 4 and length: 24, items: [(<class '__main__.MemHogger'>, 'MemHogger(5)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(5)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 7': 20mb ->->
  <- Memory after  'Creating MemHogger 7': 21mb <-
Iteration - 8 Unreachable Objects: 4 and length: 28, items: [(<class '__main__.MemHogger'>, 'MemHogger(6)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(6)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 8': 21mb ->->
  <- Memory after  'Creating MemHogger 8': 22mb <-
Iteration - 9 Unreachable Objects: 4 and length: 32, items: [(<class '__main__.MemHogger'>, 'MemHogger(7)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(7)'), (<class 'dict'>, ['hogger', 'megabyte'])]
->-> Memory before 'Creating MemHogger 9': 22mb ->->
  <- Memory after  'Creating MemHogger 9': 23mb <-
Iteration - 10 Unreachable Objects: 4 and length: 36, items: [(<class '__main__.MemHogger'>, 'MemHogger(8)'), (<class 'dict'>, ['index', 'container']), (<class '__main__.MemContainer'>, 'MemContainer(1.0mb)->MemHogger(8)'), (<class 'dict'>, ['hogger', 'megabyte'])]
==================Unreachable Objects: 4==================
40
  <- Memory after  'main run': 23mb <-

While the rss also accounts for python's internal memory allocations, it can be observed that in most iterations the rss increases in 1MB increments, as that is (roughly) MemContainer's instance size.

When excluding the SAVEALL setting (ie gc.set_debug(gc.DEBUG_UNCOLLECTABLE) ) unreachable objects get released, so while 4 objects are sill found, gc.garbage remains empty, and the output shows no dramatic increase in rss :

->-> Memory before 'main run': 10mb ->->
->-> Memory before 'Creating MemHogger 0': 10mb ->->
  <- Memory after  'Creating MemHogger 0': 11mb <-
Iteration - 1 Unreachable Objects: 0 and length: 0, items: []
->-> Memory before 'Creating MemHogger 1': 11mb ->->
  <- Memory after  'Creating MemHogger 1': 14mb <-
Iteration - 2 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 2': 13mb ->->
  <- Memory after  'Creating MemHogger 2': 15mb <-
Iteration - 3 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 3': 12mb ->->
  <- Memory after  'Creating MemHogger 3': 14mb <-
Iteration - 4 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 4': 14mb ->->
  <- Memory after  'Creating MemHogger 4': 14mb <-
Iteration - 5 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 5': 14mb ->->
  <- Memory after  'Creating MemHogger 5': 14mb <-
Iteration - 6 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 6': 14mb ->->
  <- Memory after  'Creating MemHogger 6': 14mb <-
Iteration - 7 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 7': 14mb ->->
  <- Memory after  'Creating MemHogger 7': 14mb <-
Iteration - 8 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 8': 14mb ->->
  <- Memory after  'Creating MemHogger 8': 14mb <-
Iteration - 9 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 9': 14mb ->->
  <- Memory after  'Creating MemHogger 9': 14mb <-
Iteration - 10 Unreachable Objects: 4 and length: 0, items: []
==================Unreachable Objects: 4==================
0
  <- Memory after  'main run': 14mb <-

Setting ITERATIONS = 10000 yields similar results:

->-> Memory before 'main run': 10mb ->->
->-> Memory before 'Creating MemHogger 0': 10mb ->->
  <- Memory after  'Creating MemHogger 0': 11mb <-
Iteration - 1 Unreachable Objects: 0 and length: 0, items: []
->-> Memory before 'Creating MemHogger 1': 11mb ->->
  <- Memory after  'Creating MemHogger 1': 14mb <-
Iteration - 2 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 2': 13mb ->->
  <- Memory after  'Creating MemHogger 2': 15mb <-
Iteration - 3 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 3': 11mb ->->
  <- Memory after  'Creating MemHogger 3': 14mb <-
Iteration - 4 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 4': 14mb ->->
  <- Memory after  'Creating MemHogger 4': 14mb <-
Iteration - 5 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 5': 14mb ->->
  <- Memory after  'Creating MemHogger 5': 14mb <-
Iteration - 6 Unreachable Objects: 4 and length: 0, items: []
...
->-> Memory before 'Creating MemHogger 11': 14mb ->->
  <- Memory after  'Creating MemHogger 11': 15mb <-
Iteration - 12 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 12': 15mb ->->
  <- Memory after  'Creating MemHogger 12': 15mb <-
Iteration - 13 Unreachable Objects: 4 and length: 0, items: []
...
->-> Memory before 'Creating MemHogger 101': 15mb ->->
  <- Memory after  'Creating MemHogger 101': 16mb <-
Iteration - 102 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 102': 16mb ->->
  <- Memory after  'Creating MemHogger 102': 16mb <-
Iteration - 103 Unreachable Objects: 4 and length: 0, items: []
...
->-> Memory before 'Creating MemHogger 1001': 16mb ->->
  <- Memory after  'Creating MemHogger 1001': 17mb <-
Iteration - 1002 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 1002': 17mb ->->
  <- Memory after  'Creating MemHogger 1002': 17mb <-
Iteration - 1003 Unreachable Objects: 4 and length: 0, items: []
...
Iteration - 9995 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 9995': 17mb ->->
  <- Memory after  'Creating MemHogger 9995': 17mb <-
Iteration - 9996 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 9996': 17mb ->->
  <- Memory after  'Creating MemHogger 9996': 17mb <-
Iteration - 9997 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 9997': 17mb ->->
  <- Memory after  'Creating MemHogger 9997': 17mb <-
Iteration - 9998 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 9998': 17mb ->->
  <- Memory after  'Creating MemHogger 9998': 17mb <-
Iteration - 9999 Unreachable Objects: 4 and length: 0, items: []
->-> Memory before 'Creating MemHogger 9999': 17mb ->->
  <- Memory after  'Creating MemHogger 9999': 17mb <-
Iteration - 10000 Unreachable Objects: 4 and length: 0, items: []
==================Unreachable Objects: 4==================
0
  <- Memory after  'main run': 17mb <-