Any issues with large numbers of critical sections?

Question

I have a large array of structures, like this:

typedef struct
{
    int a;
    int b;
    int c;
    etc...
}
data_type;

data_type data[100000];

I have a bunch of separate threads, each of which will want to make alterations to elements within data[]. I need to make sure that no to threads attempt to access the same data element at the same time. To be precise: one thread performing data[475].a = 3; and another thread performing data[475].b = 7; at the same time is not allowed, but one thread performing data[475].a = 3; while another thread performs data[476].a = 7; is allowed. The program is highly speed critical. My plan is to make a separate critical section for each data element like so:

typedef struct
{
    CRITICAL_SECTION critsec;
    int a;
    int b;
    int c;
    etc...
}
data_type;

In one way I guess it should all work and I should have no real questions, but not having had much experience in multithreaded programming I am just feeling a little uneasy about having so many critical sections. I'm wondering if the sheer number of them could be creating some sort of inefficiency. I'm also wondering if perhaps some other multithreading technique could be faster? Should I just relax and go ahead with plan A?

Answer 1

With this many objects, most of their critical sections will be unlocked, and there will be almost no contention. As you already know (other comment), critical sections don't require a kernel-mode transition if they're unowned. That makes critical sections efficient for this situation.

The only other consideration would be whether you would want the critical sections inside your objects or in another array. Locality of reference is a good reason to put the critical sections inside the object. When you've entered the critical section, an entire cacheline (e.g. 16 or 32 bytes) will be in memory. With a bit of padding, you can make sure each object starts on a cacheline. As a result, the object will be (partially) in cache once its critical section is entered.