CODEWITHSUNDEEP
c#.netmultithreadingparallel-processing
Sumit Gupta
Sumit Gupta

Reputation: 2192

Parallel Programing with Threads

Okay, I am bit confuse on what and how should I do. I know the theory of Parallel Programming and Threading, but here is my case:

We have number of log files in given folder. We read these log files in database. Usually reading these files take couple of hours to read, as we do it in serial method, i.e. we iterate through each file, then open a SQL transaction for each file and insert the log in database, then read another and do the same.

Now, I am thinking of using Parallel programming so I can consume all core of CPU, however I am still not clear if I use Thread for each file, will that make any difference to system? I mean if I create say 30 threads then will they run on single core or they run on Parallel ? How can I use both of them? if they are not already doing that?

EDIT: I am using Single Server, with 10K HDD Speed, and 4 Core CPU, with 4 GB RAM, no network operation, SQL Server is on same machine with Windows 2008 as OS. [can change OS if that help too :)].

EDIT 2: I run some test to be sure based on your feedbacks, here is what I found on my i3 Quad Core CPU with 4 GB RAM

  1. CPU remains at 24-50% CPU1, CPU2 remain under 50% usage, CPU3 remain at 75% usage and CPU4 remains around 0%. Yes I have Visual studio, eamil client and lot of other application open, but this tell me that application is not using all core, as CPU4 remain 0%;

  2. RAM remain constantly at 74% [it was around 50% before test], that is how we design the read. So, nothing to worry

  3. HDD remain READ/Write or usage value remain less than 25% and even it spike to 25% in sine wave, as our SQL transaction first stored in memory and then it write to disk when memory is getting threshold, so again,

So all resources are under utilized here, and hence I think I can distribute work to make it efficient. Your thoughts again. Thanks.

Upvotes: 1

Views: 853

Answers (6)

svick
svick

Reputation: 245008

First of all, you need to understand your code and why is it slow. If you're thinking something like “my code is slow and uses one CPU, so I'll just make it use all 4 CPUs and it will be 4 times faster”, then you're most likely wrong.

Using multiple threads makes sense if:

  1. Your code (or at least a part of it) is CPU bound. That is, it's not slowed down by your disk, your network connection or your database server, it's slowed down by your CPU.
  2. Or your code has multiple parts, each using a different resource. E.g. one part reads from a disk, another part converts the data, which requires lots of CPU and last part writes the data to a remote database. (Parallelizing this doesn't actually require multiple threads, but it's usually the simplest way to do it.)

From your description, it sounds like you could be in situation #2. A good solution for that is the producer consumer pattern: Stage 1 thread reads the data from the disk and puts it into a queue. Stage 2 thread takes the data from the queue, processes them and puts them into another queue. Stage 3 thread takes the processed data from the second queue and saves them to the database.

In .Net 4.0, you would use BlockingCollection<T> for the queue between the threads. And when I say “thread”, I pretty much mean Task. In .Net 4.5, you could use blocks from TPL Dataflow instead of the threads.

If you do it this way then you can get up to three times faster execution (if each stage takes the same time). If Stage 2 is the slowest part, then you can get another speedup by using more than one thread for that stage (since it's CPU bound). The same could also apply to Stage 3, depending on your network connection and your database.

Upvotes: 2

BLoB
BLoB

Reputation: 9725

Using the latest constructs in .Net 4 for parallel programming, threads are generally managed for you... take a read of getting started with parallel programming (pretty much the same as what has happened more recently with async versions of functions to use if you want it async)

e.g.

for (int i = 2; i < 20; i++)
{
    var result = SumRootN(i);
    Console.WriteLine("root {0} : {1} ", i, result);
}

becomes

Parallel.For(2, 20, (i) =>
{
    var result = SumRootN(i);
    Console.WriteLine("root {0} : {1} ", i, result);
});

EDIT: That said, it would be productive / faster to perhaps also put intensive tasks into seperate threads... but to manually make your application 'Multi-Core' and have things like certain threads running on particular cores, that isn't currently possible, that's all managed under the hood...

have a look at plinq for example and .Net Parallel Extensions and look into

System.Diagnostics.Process.GetCurrentProcess().ProcessorAffinity = 4

Edit2: Parallel processing can be done inside a single core with multiple threads.

Multi-Core processing means distributing those threads to make use of the multiple cores in a CPU.

Upvotes: 1

Pragmateek
Pragmateek

Reputation: 13396

There is no definite answer to this question and you'll have to test because as mentionned in my comments:

  • if the bottleneck is the disk I/O then you won't gain a lot by adding more threads and you might even worsen performance because more threads will be fighting to get access to the disk

  • if you think disk I/O is OK but CPU loads is the issue then you can add some threads, but no more than the number of cores because here again things will worsen due to context switching

  • if you can do more disk and network I/Os and CPU load is not high (very likely) then you can oversubscribe with (far) more threads than cores: typically if your threads are spending much of their time waiting for the database

So you should profile first, and then (or directly if you're in a hurry) test different configurations, but chances are you'll be in the third case. :)

Upvotes: 2

CodeCamper
CodeCamper

Reputation: 6984

It varies from machine to machine but speaking generally if you have a dual core processor and you have 2 threads the Operating System will pass one thread to one core and the other thread to the other. It doesn't matter how many cores you use what matters is whether your equation is the fastest. If you want to make use of Parallel programming you need a way of sharing the workload in a way that logically makes sense. Also you need to consider where your bottleneck is actually occurring. Depending on the size of the file it may be simply the max speed of your read/write of the storage medium that is taking so long.As a test I suggest you log where the most time in your code is being consumed.

A simple way to test whether a non-serial approach will help you is to sort your files in some order divide the workload between 2 threads doing the same job simultaneously and see if it makes a difference. If a second thread doesn't help you then I guarantee 30 threads will only make it take longer due to the OS having to switch threads back and fourth.

Upvotes: 1

Mzf
Mzf

Reputation: 5260

Your problem is not using all CPU, your action are mainly I/O (reading file , sending data to DB).

Using Thread/Parallel will make your code run faster since you are processing many files at the same time.

To answer your question , the framework/OS will optimize running your code over the different cores.

Upvotes: 1

nvoigt
nvoigt

Reputation: 77364

First, you should check what is taking the time. If the CPU actually is the bottleneck, parallel processing will help. Maybe it's the network and a faster network connection will help. Maybe buying a faster disc will help.

Find the problem before thinking about a solution.

Upvotes: 1

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