MAX_BACKENDS size (comment accuracy)
| От | Jacob Brazeal |
|---|---|
| Тема | MAX_BACKENDS size (comment accuracy) |
| Дата | |
| Msg-id | CA+COZaBO_s3LfALq=b+HcBHFSOEGiApVjrRacCe4VP9m7CJsNQ@mail.gmail.com обсуждение исходный текст |
| Список | pgsql-hackers |
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))
#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
* 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
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