Thinking a bit further about this, the purpose of the LW_SHARED_MASK section of the state is to count the number of lock-sharers. Thus, we only care about the actual number of backends (up to 2^18-1) here and not the size of the ProcNumber data type. So I do think the comment should read 2^18-1 and not 2^23-1. Here is a patch to that effect.
On Sat, Jan 25, 2025 at 3:21 PM Jacob Brazeal <jacob.braz...@gmail.com> wrote: > Hello all, > > In lwlocks.c, we have the following comment, related to LWLock state: > > > */* Must be greater than MAX_BACKENDS - which is 2^23-1, so we're fine. > */#define LW_SHARED_MASK ((uint32) ((1 << 24)-1))* > > However, MAX_BACKENDS is set to 2^18-1. Here is the comment in > postmaster.h: > > > > > > > > > > > > */* * Note: MAX_BACKENDS is limited to 2^18-1 because that's the width > reserved * for buffer references in buf_internals.h. This limitation could > be lifted * by using a 64bit state; but it's unlikely to be worthwhile as > 2^18-1 * backends exceed currently realistic configurations. Even if that > limitation * were removed, we still could not a) exceed 2^23-1 because > inval.c stores * the ProcNumber as a 3-byte signed integer, b) INT_MAX/4 > because some places * compute 4*MaxBackends without any overflow check. > This is rechecked in the * relevant GUC check hooks and in > RegisterBackgroundWorker(). */#define MAX_BACKENDS 0x3FFFF* > > 2^23-1 is noted as an additional upper limit, but I wonder if it'd be > correct to update the comment in lwlocks.c to > > > */* Must be greater than MAX_BACKENDS - which is 2^18-1, so we're fine. */* > > I'm not sure if this could lead to us actually saving some bits in the > lwlock state, or if we could do anything useful with them anyway. > > Jacob >
v0-backend-size.patch
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