Re: GC.Collect

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Hi All ,

Thanks for the suggestions / pointers , they will definitely help me in making a wise decision .

just to divulge some more details , i am the developer of the stress test suite , which is stressing another app. and checking it's performance under load , where for around 3 hrs. , all 200 threads keep on going in loop , with a sleep time of 1sec. after every iteration , but what i make sure is :

1. No major allocations are done in that loop , essntially not a very heavy work per iteration .

2. Before leaving the loop , relevant objects are closed , disposed and nullified , so that's why i never thought it to be necessary to go in collect method .

but on other end App. developer doesn't want to take a blame of some arrant mem. leak / handle leak , so he's bent upon using collect and waitforpendingfinalizers in all iterations , however i don't think by doing this i am simulating a real scenario .

any more suggestions , most welcome .

thanks ,

Mrinal

Michael S wrote:

"Mrinal Kamboj" <mrinal.kamboj@xxxxxxxxxx> wrote in message news:%A6Ge.21$lz1.108@xxxxxxxxxxxxxxxxxx

Hi ,

Any pointers when it's absolute necessary to use it . 


I'll try give you a few.. But first a few rules

Rule 1) Never assume you know how many generations .NET uses or their sizes. (Sure we have 3 of them and gen0 is 256K, but this may change in future systems and differ on platforms)

Rule 2) The GC knows best when to collect. (It keeps stats for gen2 and collect at will, while gen1 and gen0 gets collected when full. This may change in future systems.)

However. There are cases when you may want to collect. These could be:

1) When you are in a huge complex (i.e nested) loops and know you are allocating plenty of reference-types that is fire and forget (read strings). Hence they won't survive the outer loop but exist in the inner. I'm talking about if you got O(N^3) and know that the code will allocate several thousands or millions of objects. Well for each 65'000 of those objects the gen0 in .NET 1.1 will get full, the GC kicks in and move most of them to gen1. And when gen1 gets full the'll be moved to gen2. This is not what you want for your short lived objects and you can help the GC to collect. By collecting often you leave more space for other objects.

2) When using COM. If you automate Excel or Word, the marshalling generates a lot of objects that tends to stick and move to gen1 and gen2 really fast. If you write a lot of data to excel it is just fair to do a collect when you're done with the filling.

3) When .NET is eating memory like a hog and you don't know why. A GC.Collect() is a great debugging-tool for finding poor performance. Just move that line around in the code until .NET stops eating memory like Godzilla chews up Tokyo and you just found a place to refactor. =)



Does it has a blocking effect on the code , as GC per se is undeterministic .


Yes. When the GC kicks in it will block just about everything,one application domain at a time. But the GC lives in his own thread and you will hardly notice a gen0 collect. But if gen2 have grown to giga-size and start collect and re-arrange you will feel a stagger and winamp may lag. =)

This is why you might wanna help the GC sometimes.


what if GC.collect is followed in next line by GC.WaitForPendingFinalizers , will it actually block . 


Yes. or else it would imply that the GC was buggy.


Why i need all this info as in my stress test suite developer is stressing upon the need to use these things in every iteration of every thread , where simultaneously around 200 worker thread are working , so i am not very convinced about such a high frequency usage .


Well, 200 threads is quite a lot for one application. And as you are stressing another app, you are stressing your dear .NET runtime with context-switching. Hence this would be an excellent case for when you want to help the GC to collect.

But then again, for most applications there is no need to collect. Rule 1 and 2 applies.

Happy Collecting
- Michael S



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