Utilizing idle socket server to do meaningful thing (timeout after sleep on epoll_wait)
Keeping the cache hot

So my initiative is to make the sleep time short and force the process to read from memory again and again until the events come.
Reason for doing this is that to keep critical (for performance) data hot in the cache.

If nothing else running on the same CPU, there is no reason for the cache to go cold. Things in the cache don't time out, they just stay there until they are evicted when necessary.
If there are other processes running, then your strategy might help your socket server, but there are several issues with this:

It is selfish; you might prevent other processes from keeping their data in the cache, thus lowering their performance.
It might not work at all, since once another process gets a time slice, their memory access patterns may evict your data from the cache again.
By not staying idle, your CPU might not get a chance to go into a low power state, thus keeping your cache literally hot.

My recommendation is to just use an infinite timeout for epoll_wait().
About memcmp_explicit()
Be aware that compilers are getting better and better at optimizing things. Even if you put memcmp_explicit() in a different translation unit than where it is called, the compiler might optimize it out if link time optimization is enabled.