Question

I have been given the task of re-writing some libraries written in C# so that there are no allocations once startup is completed.

I just got to one project that does some DB queries over an OdbcConnection every 30 seconds. I’ve always just used .ExecuteReader() which creates an OdbcDataReader. Is there any pattern (like the SocketAsyncEventArgs socket pattern) that lets you re-use your own OdbcDataReader? Or some other clever way to avoid allocations?

I haven’t bothered to learn LINQ since all the dbs at work are Oracle based and the last I checked, there was no official Linq To Oracle provider. But if there’s a way to do this in Linq, I could use one of the third-party ones.

Update:

I don’t think I clearly specified the reasons for the no-alloc requirement. We have one critical thread running and it is very important that it not freeze. This is for a near realtime trading application, and we do see up to a 100 ms freeze for some Gen 2 collections. (I’ve also heard of games being written the same way in C#). There is one background thread that does some compliance checking and runs every 30 seconds. It does a db query right now. The query is quite slow (approx 500 ms to return with all the data), but that is okay because it doesn’t interfere with the critical thread. Except if the worker thread is allocating memory, it will cause GCs which freeze all threads.

I’ve been told that all the libraries (including this one) cannot allocate memory after startup. Whether I agree with that or not, that’s the requirement from the people who sign the checks :).

Now, clearly there are ways that I could get the data into this process without allocations. I could set up another process and connect it to this one using a socket. The new .NET 3.5 sockets were specifically optimized not to allocate at all, using the new SocketAsyncEventArgs pattern. (In fact, we are using them to connect to several systems and never see any GCs from them.) Then have a pre-allocated byte array that reads from the socket and go through the data, allocating no strings along the way. (I’m not familiar with other forms of IPC in .NET so I’m not sure if the memory mapped files and named pipes allocate or not).

But if there’s a faster way to get this no-alloc query done without going through all that hassle, I’d prefer it.

Answer 1

You cannot reuse IDataReader (or OdbcDataReader or SqlDataReader or any equivalent class). They are designed to be used with a single query only. These objects encapsulate a single record set, so once you’ve obtained and iterated it, it has no meaning anymore.

Creating a data reader is an incredibly cheap operation anyway, vanishingly small in contrast to the cost of actually executing the query. I cannot see a logical reason for this “no allocations” requirement.

I’d go so far as to say that it’s very nearly impossible to rewrite a library so as to allocate no memory. Even something as simple as boxing an integer or using a string variable is going to allocate some memory. Even if it were somehow possible to reuse the reader (which it isn’t, as I explained), it would still have to issue the query to the database again, which would require memory allocations in the form of preparing the query, sending it over the network, retrieving the results again, etc.

Avoiding memory allocations is simply not a practical goal. Better to perhaps avoid specific types of memory allocations if and when you determine that some specific operation is using up too much memory.