How to handle mass database manipulation every second - threading? - performance

Question

I have a very hard problem:
I have round about 20-50 objects, which I MUST (that is given for the problem, please don't spend time in thinking around it) put througt a logic EVERY SECOND.
The logic itself need round about 200-600 milliseconds (90% it is 200ms - 10% it is 600ms).
I try to find any solution how I can make is smaller, but there isn't. I must get an object from DB, I must have a lot of if-else and I must actual it. - Even if I reduce it to 50ms or smaller, to veriable rate of the object up to 50 will break my neck with the 1 second timer, because 50 x 50mx =2,5 second. So a tick needs longer then the tickrate should be.
So, my only, not very smart I think, idea is to open for every object an own thread and lead a mainthread for handling. So the mainthread opens x other thread. So only this opening must take unter 1 second. After it logic is used, the thread can kill itself and we all are happy, aren't we?
By given the last answers, I will explain my problem:
I try to build an auctioneer site. So I have up to 50 auctions running at the same moment - nothing special. So I need to: every single second look to the auctionlist, see if the time is 00:00:01 and if it is, bid automaticly (it's a feature, that user can create).
So: get 50 objects in a list, iterate through, check if a automatic bid is need, do it.

Answer 1

With 50 objects and the processing time you've given on average you are doing 12 seconds worth of processing every second. Assuming you have 4 cores, you can get this down to an execution time of 4 seconds via threading. Every second. This means that you're going to start off behind and slip further behind as time goes on.
I know you said you tried to think of a way to make it more efficient, but couldn't, but I fear you're going to have to. The problem as stated now is computationally intractable. You're either going to have to process the objects in a rotating window (so each object gets hit once every 4th cycle or so), or you need to make your processing run faster.

Answer 2

First: Profile, if you haven't already. Figure out what section of your code are taking time, etc. I'd go after that database - how long is the I/O of the objects from the database taking? Can you cache that I/O? (If you're manipulating the same 50 objects, don't load them every second.)
Let's address your threads idea: If you want multiple threads, don't create and destroy them every second. Create your X threads, and leave them be -- creating & destroying them are going to be expensive operations. You might find that less threads will work better - such as 1 or 2 per core, as you might be able to reduce time doing context switches.

Answer 3

To expand on Jonathan Leffler's comment on the question, as the OP requested: (This answer is a wiki)
Say you have these three things being auctioned, ending at the times indicated:
10 Apples - ends at 1:05:00 PM
20 Blueberries - ends at 2:00:00 PM
15 Pears - ends at 3:50:00 PM
If the current time is 1:00:00 PM, then sleep for 4 minutes, 58 seconds (since the closest item ends in 5 minutes). We use the 2 seconds then for processing - adjust that threshold as needed. Once we're done with the apples, we'll sleep for (2 PM - now() - 2s), for the blueberries.
Note that when we wake up at 1:04:58 PM to process the apples auction, we do not touch the blueberries or the pears -- we know that they're still way out in the future, so we don't care.