I am using multiprocessing.Pool in python to schedule around 2500 jobs. I am submitting the jobs like this:
pool = multiprocessing.Pool()
for i from 1 to 2500: # pseudocode
jobs.append(pool.apply_async(....))
for j in jobs:
_ = job.get()
The jobs are such, that after some computation, they go to sleep for a long time, waiting for some event to complete. My expectation was that, while they sleep, the other waiting jobs would get scheduled. But it is not happening like that. The maximum number of jobs scheduled at a single time is around 23 (even though they are all sleeping, ps aux shows state S+) which is the more or less the number of cores in the machine. Only after a job finishes and releases a core, another job is getting scheduled.
My expectation was that all 2500 jobs would get scheduled at once. How do I make python submit all 2500 jobs at once?
The multiprocessing and threading package of Python use process/thread pools. By default the number of processes/threads in a pool is dependent of the hardware concurrency (ie. typically the number of hardware threads supported by your processor). You can tune this number but you should really not create too many threads or processes because they are precious resources of the operating system (OS). Note that threads are less expensive than processes for most OS but the CPython makes threads not very useful (except of IO latency-bound jobs) because of the global interpreter lock (GIL). Creating 2500 processes/threads will put a lot of pressure on the OS scheduler and slow does the whole system. OS are design so that waiting threads are not expensive but frequent wake ups will be clearly expensive. Moreover, the number of processes/threads that can be created on a given platform is bounded. AFAIR, on my old Windows 7 system this was limited to 1024. The biggest problem is that each thread requires a stack typically initialized to 1~2 MiB so that creating 2500 threads will takes 2.5~5.0 GiB of RAM! This will be significantly worst for processes. Not to mention cache misses will be more frequent resulting in a slower execution. Thus, put it shortly, do not create 2500 threads or processes: this is too expensive.
You do not need threads or processes, you needs fibers or more generally green threads likes greenlet or eventlet as well as gevent coroutines. The last is known to be fast and supports thread-pools. Alternatively, you can use the recent async feature of Python which is the standard way to deal with such a problem.
pool = multiprocessing.Pool() will use the all available cores. If you need to use a specific number of processes you need to specify it as argument.
pool = multiprocessing.Pool(processes=100)
When creating a multiprocessing.Pool instance with no argument, it is my understanding that the pool size will be assigned the value of os.cpu_count()
On my machine os.cpu_count() == 20
Also:-
from multiprocessing import Pool
with Pool() as pool:
print(len(pool.__dict__['_pool']))
...gives me a value of 20
But here's the problem (if indeed it is a problem). My CPU is a Xeon W-2150B. Thus it is a single CPU with 10 cores where each core is capable of handling 2 concurrent threads (hyper-threading). And so the value of 20 seems to be the number of concurrent threads that the CPU can handle.
However, it seems to me that one wouldn't want to create a pool size that's going to "use up" the CPU's capabilities in its entirety because any other processes running on the same machine would suffer degradation.
So, my thinking is that the pool size should be limited to (number_of_cores - 1)
One way of doing this would be as follows:-
import psutil
from multiprocessing import Pool
maxWorkers = max(2, psutil.cpu_count(logical=False) - 1)
with Pool(maxWorkers) as pool:
print(len(pool.__dict__['_pool']))
...gives a value of 9 which I think is much more reasonable and, potentially, more portable.
Is this a reasonable approach?
It depends on your application.
When you create a Pool in size n, n empty processes are created. Those processes almost doesn't consume CPU until a task is applied to them.
So another approach can be to set the Pool with maximum size and limit the number of running processes with another logic - hold a queue for incoming tasks, and apply them to the pool as long as you don't reach a limit of parallelly running processes.
This approach might be useful if you might want to increase the number of running processes in runtime. Decreasing the number of processes needs more logic.
Notice that even after you apply a task to a process in the pool, the OS still have control over it, so it can be scheduled and interrupted by the OS according to its priority.
I have a dictionary (in memory) data that has ~ 10,000 keys which each key represent a stock ticker, and the value stores the pandas dataframe representation of time series data for daily stock price. I am trying to calculate the pairwise Pearson correlation.
The code takes a long time ~3 hr to fully iterate through all the combinations O(n^2) ~ C(2, 10000). I tried to use multiprocessing dummy package but saw no performance gain AT ALL (actually slower as the number of workers increases).
from multiprocessing.dummy import Pool
def calculate_correlation((t1, t2)):
# pseudo code here
return pearsonr(data[t1]['Close'], data[t2]['Close'])
todos = []
for idx, t1 in enumerate(list(data.keys())):
for t2 in list(data.keys())[idx:]: # only the matrix top triangle
todos.append((t1, t2))
pool = Pool(4)
results = pool.map(calculate_correlation, todos)
pool.close()
pool.join()
All the data has been loaded into memory so it should not be IO intensive. Is there any reason that why there is no performance gain at all?
When you use multiprocessing.dummy, you're using threads, not processes. For a CPU-bound application in Python, you are usually not going to get performance boost when using multi-threading. You should use multi-processing instead to parallelize your code in Python. So, if you change your code from
from multiprocessing.dummy import Pool
to
from multiprocessing import Pool
This should substantially improve your performance.
The above will fix your problem, but if you want to know why this happened. Please continue reading:
Multi-threading in Python has Global Interpreter Lock (GIL) that prevents two threads in the same process to run at the same time. If you had a a lot of disk IO happening, multi-threading would have helped because DISK IO is separate process that can handle locks. Or, if you had a separate application used by your Python code that can handle locks, multi-threading would have helped. Multi-processing, on the other hand, will use all the cores of your CPU as separate processes as opposed to multi-threading. In CPU bound Python application such as yours, if you use multi-processing instead of multi-threading, your application will run on multiple processes on several cores in parallel which will boost the performance of your application.
I've to process say thousands of records in an array. I did the normal for loop like this
for record in records:
results = processFile(record)
write_output_record(o, results)
The script above took 427.270612955 seconds!
As there is no dependancy between these records. I used Python multi threading module in a hope to speedup the process. below is my implementation
import multiprocessing
from multiprocessing.dummy import Pool as ThreadPool
pool = ThreadPool(processes=threads)
results = pool.map(processFile, records)
pool.close()
pool.join()
write_output(o, results)
My computer has 8 cpu's. And it takes 852.153398991 second.
Can somebody help me as in what am I doing wrong?
PS: processFile function has no i/o's. its mostly processing the records and sending back the update record
Try using vmstat and verify whether its a memory issue. Sometimes, using multithreading can slow your system down if each thread pushes up the RAM usage by a significant amount.
Usually people encounter three types of issues: CPU bound (Constraint on CPU computations), Memory bound (Constraint on RAM) and I/O bound (Network & hard drive I/O constraints).
This question already has answers here:
What are the differences between the threading and multiprocessing modules?
(6 answers)
Closed 3 years ago.
I am trying to understand the advantages of multiprocessing over threading. I know that multiprocessing gets around the Global Interpreter Lock, but what other advantages are there, and can threading not do the same thing?
Here are some pros/cons I came up with.
Multiprocessing
Pros
Separate memory space
Code is usually straightforward
Takes advantage of multiple CPUs & cores
Avoids GIL limitations for cPython
Eliminates most needs for synchronization primitives unless if you use shared memory (instead, it's more of a communication model for IPC)
Child processes are interruptible/killable
Python multiprocessing module includes useful abstractions with an interface much like threading.Thread
A must with cPython for CPU-bound processing
Cons
IPC a little more complicated with more overhead (communication model vs. shared memory/objects)
Larger memory footprint
Threading
Pros
Lightweight - low memory footprint
Shared memory - makes access to state from another context easier
Allows you to easily make responsive UIs
cPython C extension modules that properly release the GIL will run in parallel
Great option for I/O-bound applications
Cons
cPython - subject to the GIL
Not interruptible/killable
If not following a command queue/message pump model (using the Queue module), then manual use of synchronization primitives become a necessity (decisions are needed for the granularity of locking)
Code is usually harder to understand and to get right - the potential for race conditions increases dramatically
The threading module uses threads, the multiprocessing module uses processes. The difference is that threads run in the same memory space, while processes have separate memory. This makes it a bit harder to share objects between processes with multiprocessing. Since threads use the same memory, precautions have to be taken or two threads will write to the same memory at the same time. This is what the global interpreter lock is for.
Spawning processes is a bit slower than spawning threads.
Threading's job is to enable applications to be responsive. Suppose you have a database connection and you need to respond to user input. Without threading, if the database connection is busy the application will not be able to respond to the user. By splitting off the database connection into a separate thread you can make the application more responsive. Also because both threads are in the same process, they can access the same data structures - good performance, plus a flexible software design.
Note that due to the GIL the app isn't actually doing two things at once, but what we've done is put the resource lock on the database into a separate thread so that CPU time can be switched between it and the user interaction. CPU time gets rationed out between the threads.
Multiprocessing is for times when you really do want more than one thing to be done at any given time. Suppose your application needs to connect to 6 databases and perform a complex matrix transformation on each dataset. Putting each job in a separate thread might help a little because when one connection is idle another one could get some CPU time, but the processing would not be done in parallel because the GIL means that you're only ever using the resources of one CPU. By putting each job in a Multiprocessing process, each can run on its own CPU and run at full efficiency.
Python documentation quotes
The canonical version of this answer is now at the dupliquee question: What are the differences between the threading and multiprocessing modules?
I've highlighted the key Python documentation quotes about Process vs Threads and the GIL at: What is the global interpreter lock (GIL) in CPython?
Process vs thread experiments
I did a bit of benchmarking in order to show the difference more concretely.
In the benchmark, I timed CPU and IO bound work for various numbers of threads on an 8 hyperthread CPU. The work supplied per thread is always the same, such that more threads means more total work supplied.
The results were:
Plot data.
Conclusions:
for CPU bound work, multiprocessing is always faster, presumably due to the GIL
for IO bound work. both are exactly the same speed
threads only scale up to about 4x instead of the expected 8x since I'm on an 8 hyperthread machine.
Contrast that with a C POSIX CPU-bound work which reaches the expected 8x speedup: What do 'real', 'user' and 'sys' mean in the output of time(1)?
TODO: I don't know the reason for this, there must be other Python inefficiencies coming into play.
Test code:
#!/usr/bin/env python3
import multiprocessing
import threading
import time
import sys
def cpu_func(result, niters):
'''
A useless CPU bound function.
'''
for i in range(niters):
result = (result * result * i + 2 * result * i * i + 3) % 10000000
return result
class CpuThread(threading.Thread):
def __init__(self, niters):
super().__init__()
self.niters = niters
self.result = 1
def run(self):
self.result = cpu_func(self.result, self.niters)
class CpuProcess(multiprocessing.Process):
def __init__(self, niters):
super().__init__()
self.niters = niters
self.result = 1
def run(self):
self.result = cpu_func(self.result, self.niters)
class IoThread(threading.Thread):
def __init__(self, sleep):
super().__init__()
self.sleep = sleep
self.result = self.sleep
def run(self):
time.sleep(self.sleep)
class IoProcess(multiprocessing.Process):
def __init__(self, sleep):
super().__init__()
self.sleep = sleep
self.result = self.sleep
def run(self):
time.sleep(self.sleep)
if __name__ == '__main__':
cpu_n_iters = int(sys.argv[1])
sleep = 1
cpu_count = multiprocessing.cpu_count()
input_params = [
(CpuThread, cpu_n_iters),
(CpuProcess, cpu_n_iters),
(IoThread, sleep),
(IoProcess, sleep),
]
header = ['nthreads']
for thread_class, _ in input_params:
header.append(thread_class.__name__)
print(' '.join(header))
for nthreads in range(1, 2 * cpu_count):
results = [nthreads]
for thread_class, work_size in input_params:
start_time = time.time()
threads = []
for i in range(nthreads):
thread = thread_class(work_size)
threads.append(thread)
thread.start()
for i, thread in enumerate(threads):
thread.join()
results.append(time.time() - start_time)
print(' '.join('{:.6e}'.format(result) for result in results))
GitHub upstream + plotting code on same directory.
Tested on Ubuntu 18.10, Python 3.6.7, in a Lenovo ThinkPad P51 laptop with CPU: Intel Core i7-7820HQ CPU (4 cores / 8 threads), RAM: 2x Samsung M471A2K43BB1-CRC (2x 16GiB), SSD: Samsung MZVLB512HAJQ-000L7 (3,000 MB/s).
Visualize which threads are running at a given time
This post https://rohanvarma.me/GIL/ taught me that you can run a callback whenever a thread is scheduled with the target= argument of threading.Thread and the same for multiprocessing.Process.
This allows us to view exactly which thread runs at each time. When this is done, we would see something like (I made this particular graph up):
+--------------------------------------+
+ Active threads / processes +
+-----------+--------------------------------------+
|Thread 1 |******** ************ |
| 2 | ***** *************|
+-----------+--------------------------------------+
|Process 1 |*** ************** ****** **** |
| 2 |** **** ****** ** ********* **********|
+-----------+--------------------------------------+
+ Time --> +
+--------------------------------------+
which would show that:
threads are fully serialized by the GIL
processes can run in parallel
The key advantage is isolation. A crashing process won't bring down other processes, whereas a crashing thread will probably wreak havoc with other threads.
As mentioned in the question, Multiprocessing in Python is the only real way to achieve true parallelism. Multithreading cannot achieve this because the GIL prevents threads from running in parallel.
As a consequence, threading may not always be useful in Python, and in fact, may even result in worse performance depending on what you are trying to achieve. For example, if you are performing a CPU-bound task such as decompressing gzip files or 3D-rendering (anything CPU intensive) then threading may actually hinder your performance rather than help. In such a case, you would want to use Multiprocessing as only this method actually runs in parallel and will help distribute the weight of the task at hand. There could be some overhead to this since Multiprocessing involves copying the memory of a script into each subprocess which may cause issues for larger-sized applications.
However, Multithreading becomes useful when your task is IO-bound. For example, if most of your task involves waiting on API-calls, you would use Multithreading because why not start up another request in another thread while you wait, rather than have your CPU sit idly by.
TL;DR
Multithreading is concurrent and is used for IO-bound tasks
Multiprocessing achieves true parallelism and is used for CPU-bound tasks
Another thing not mentioned is that it depends on what OS you are using where speed is concerned. In Windows processes are costly so threads would be better in windows but in unix processes are faster than their windows variants so using processes in unix is much safer plus quick to spawn.
Other answers have focused more on the multithreading vs multiprocessing aspect, but in python Global Interpreter Lock (GIL) has to be taken into account. When more number (say k) of threads are created, generally they will not increase the performance by k times, as it will still be running as a single threaded application. GIL is a global lock which locks everything out and allows only single thread execution utilizing only a single core. The performance does increase in places where C extensions like numpy, Network, I/O are being used, where a lot of background work is done and GIL is released. So when threading is used, there is only a single operating system level thread while python creates pseudo-threads which are completely managed by threading itself but are essentially running as a single process. Preemption takes place between these pseudo threads. If the CPU runs at maximum capacity, you may want to switch to multiprocessing.
Now in case of self-contained instances of execution, you can instead opt for pool. But in case of overlapping data, where you may want processes communicating you should use multiprocessing.Process.
MULTIPROCESSING
Multiprocessing adds CPUs to increase computing power.
Multiple processes are executed concurrently.
Creation of a process is time-consuming and resource intensive.
Multiprocessing can be symmetric or asymmetric.
The multiprocessing library in Python uses separate memory space, multiple CPU cores, bypasses GIL limitations in CPython, child processes are killable (ex. function calls in program) and is much easier to use.
Some caveats of the module are a larger memory footprint and IPC’s a little more complicated with more overhead.
MULTITHREADING
Multithreading creates multiple threads of a single process to increase computing power.
Multiple threads of a single process are executed concurrently.
Creation of a thread is economical in both sense time and resource.
The multithreading library is lightweight, shares memory, responsible for responsive UI and is used well for I/O bound applications.
The module isn’t killable and is subject to the GIL.
Multiple threads live in the same process in the same space, each thread will do a specific task, have its own code, own stack memory, instruction pointer, and share heap memory.
If a thread has a memory leak it can damage the other threads and parent process.
Example of Multi-threading and Multiprocessing using Python
Python 3 has the facility of Launching parallel tasks. This makes our work easier.
It has for thread pooling and Process pooling.
The following gives an insight:
ThreadPoolExecutor Example
import concurrent.futures
import urllib.request
URLS = ['http://www.foxnews.com/',
'http://www.cnn.com/',
'http://europe.wsj.com/',
'http://www.bbc.co.uk/',
'http://some-made-up-domain.com/']
# Retrieve a single page and report the URL and contents
def load_url(url, timeout):
with urllib.request.urlopen(url, timeout=timeout) as conn:
return conn.read()
# We can use a with statement to ensure threads are cleaned up promptly
with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
# Start the load operations and mark each future with its URL
future_to_url = {executor.submit(load_url, url, 60): url for url in URLS}
for future in concurrent.futures.as_completed(future_to_url):
url = future_to_url[future]
try:
data = future.result()
except Exception as exc:
print('%r generated an exception: %s' % (url, exc))
else:
print('%r page is %d bytes' % (url, len(data)))
ProcessPoolExecutor
import concurrent.futures
import math
PRIMES = [
112272535095293,
112582705942171,
112272535095293,
115280095190773,
115797848077099,
1099726899285419]
def is_prime(n):
if n % 2 == 0:
return False
sqrt_n = int(math.floor(math.sqrt(n)))
for i in range(3, sqrt_n + 1, 2):
if n % i == 0:
return False
return True
def main():
with concurrent.futures.ProcessPoolExecutor() as executor:
for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
print('%d is prime: %s' % (number, prime))
if __name__ == '__main__':
main()
Threads share the same memory space to guarantee that two threads don't share the same memory location so special precautions must be taken the CPython interpreter handles this using a mechanism called GIL, or the Global Interpreter Lock
what is GIL(Just I want to Clarify GIL it's repeated above)?
In CPython, the global interpreter lock, or GIL, is a mutex that protects access to Python objects, preventing multiple threads from executing Python bytecodes at once. This lock is necessary mainly because CPython's memory management is not thread-safe.
For the main question, we can compare using Use Cases, How?
1-Use Cases for Threading: in case of GUI programs threading can be used to make the application responsive For example, in a text editing program, one thread can take care of recording the user inputs, another can be responsible for displaying the text, a third can do spell-checking, and so on. Here, the program has to wait for user interaction. which is the biggest bottleneck. Another use case for threading is programs that are IO bound or network bound, such as web-scrapers.
2-Use Cases for Multiprocessing: Multiprocessing outshines threading in cases where the program is CPU intensive and doesn’t have to do any IO or user interaction.
For More Details visit this link and link or you need in-depth knowledge for threading visit here for Multiprocessing visit here
Process may have multiple threads. These threads may share memory and are the units of execution within a process.
Processes run on the CPU, so threads are residing under each process. Processes are individual entities which run independently. If you want to share data or state between each process, you may use a memory-storage tool such as Cache(redis, memcache), Files, or a Database.
As I learnd in university most of the answers above are right. In PRACTISE on different platforms (always using python) spawning multiple threads ends up like spawning one process. The difference is the multiple cores share the load instead of only 1 core processing everything at 100%. So if you spawn for example 10 threads on a 4 core pc, you will end up getting only the 25% of the cpus power!! And if u spawn 10 processes u will end up with the cpu processing at 100% (if u dont have other limitations). Im not a expert in all the new technologies. Im answering with own real experience background