Re: refactoring relation extension and BufferAlloc(), faster COPY - Mailing list pgsql-hackers

On 01/03/2023 09:33, Andres Freund wrote:
> On 2023-02-21 17:33:31 +0100, Alvaro Herrera wrote:
>> On 2023-Feb-21, Heikki Linnakangas wrote:
>>
>>>> +static BlockNumber
>>>> +BulkExtendSharedRelationBuffered(Relation rel,
>>>> +                                 SMgrRelation smgr,
>>>> +                                 bool skip_extension_lock,
>>>> +                                 char relpersistence,
>>>> +                                 ForkNumber fork, ReadBufferMode mode,
>>>> +                                 BufferAccessStrategy strategy,
>>>> +                                 uint32 *num_pages,
>>>> +                                 uint32 num_locked_pages,
>>>> +                                 Buffer *buffers)
>>>
>>> Ugh, that's a lot of arguments, some are inputs and some are outputs. I
>>> don't have any concrete suggestions, but could we simplify this somehow?
>>> Needs a comment at least.
>>
>> Yeah, I noticed this too.  I think it would be easy enough to add a new
>> struct that can be passed as a pointer, which can be stack-allocated
>> by the caller, and which holds the input arguments that are common to
>> both functions, as is sensible.
> 
> I played a fair bit with various options. I ended up not using a struct to
> pass most options, but instead go for a flags argument. However, I did use a
> struct for passing either relation or smgr.
> 
> 
> typedef enum ExtendBufferedFlags {
>     EB_SKIP_EXTENSION_LOCK = (1 << 0),
>     EB_IN_RECOVERY = (1 << 1),
>     EB_CREATE_FORK_IF_NEEDED = (1 << 2),
>     EB_LOCK_FIRST = (1 << 3),
> 
>     /* internal flags follow */
>     EB_RELEASE_PINS = (1 << 4),
> } ExtendBufferedFlags;

Is EB_IN_RECOVERY always set when RecoveryInProgress()? Is it really 
needed? What does EB_LOCK_FIRST do?

> /*
>   * To identify the relation - either relation or smgr + relpersistence has to
>   * be specified. Used via the EB_REL()/EB_SMGR() macros below. This allows us
>   * to use the same function for both crash recovery and normal operation.
>   */
> typedef struct ExtendBufferedWhat
> {
>     Relation rel;
>     struct SMgrRelationData *smgr;
>     char relpersistence;
> } ExtendBufferedWhat;
> 
> #define EB_REL(p_rel) ((ExtendBufferedWhat){.rel = p_rel})
> /* requires use of EB_SKIP_EXTENSION_LOCK */
> #define EB_SMGR(p_smgr, p_relpersistence) ((ExtendBufferedWhat){.smgr = p_smgr, .relpersistence = p_relpersistence})

Clever. I'm still not 100% convinced we need the EB_SMGR variant, but 
with this we'll have the flexibility in any case.

> extern Buffer ExtendBufferedRel(ExtendBufferedWhat eb,
>                                 ForkNumber forkNum,
>                                 BufferAccessStrategy strategy,
>                                 uint32 flags);
> extern BlockNumber ExtendBufferedRelBy(ExtendBufferedWhat eb,
>                                        ForkNumber fork,
>                                        BufferAccessStrategy strategy,
>                                        uint32 flags,
>                                        uint32 extend_by,
>                                        Buffer *buffers,
>                                        uint32 *extended_by);
> extern Buffer ExtendBufferedRelTo(ExtendBufferedWhat eb,
>                                   ForkNumber fork,
>                                   BufferAccessStrategy strategy,
>                                   uint32 flags,
>                                   BlockNumber extend_to,
>                                   ReadBufferMode mode);
> 
> As you can see I removed ReadBufferMode from most of the functions (as
> suggested by Heikki earlier). When extending by 1/multiple pages, we only need
> to know whether to lock or not.

Ok, that's better. Still complex and a lot of arguments, but I don't 
have any great suggestions on how to improve it.

> The reason ExtendBufferedRelTo() has a 'mode' argument is that that allows to
> fall back to reading page normally if there was a concurrent relation
> extension.

Hmm, I think you'll need another return value, to let the caller know if 
the relation was extended or not. Or a flag to ereport(ERROR) if the 
page already exists, for ginbuild() and friends.

> The reason EB_CREATE_FORK_IF_NEEDED exists is to remove the duplicated,
> gnarly, code to do so from vm_extend(), fsm_extend().

Makes sense.

> I'm not sure about the function naming pattern. I do like 'By' a lot more than
> the Bulk prefix I used before.

+1

- Heikki