Re: Improve WALRead() to suck data directly from WAL buffers when possible

On Tue, Feb 28, 2023 at 10:38:31AM +0530, Bharath Rupireddy wrote:
> On Tue, Feb 28, 2023 at 6:14 AM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
>> Why do we only read a page at a time in XLogReadFromBuffersGuts()? What is
>> preventing us from copying all the data we need in one go?
>
> Note that most of the WALRead() callers request a single page of
> XLOG_BLCKSZ bytes even if the server has less or more available WAL
> pages. It's the streaming replication wal sender that can request less
> than XLOG_BLCKSZ bytes and upto MAX_SEND_SIZE (16 * XLOG_BLCKSZ). And,
> if we read, say, MAX_SEND_SIZE at once while holding
> WALBufMappingLock, that might impact concurrent inserters (at least, I
> can say it in theory) - one of the main intentions of this patch is
> not to impact inserters much.

Perhaps we should test both approaches to see if there is a noticeable
difference. It might not be great for concurrent inserts to repeatedly
take the lock, either. If there's no real difference, we might be able to
simplify the code a bit.

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com