Hi, On 2023-02-16 16:58:23 +0900, Michael Paquier wrote: > On Wed, Feb 15, 2023 at 01:00:00PM +0530, Bharath Rupireddy wrote: > > The v3 patch reduces initialization of iovec array elements which is a > > clear win when pg_pwrite_zeros is called for sizes less than BLCKSZ > > many times (I assume this is what is needed for the relation extension > > lock improvements feature). However, it increases the number of iovec > > initialization with zerobuf for the cases when pg_pwrite_zeros is > > called for sizes far greater than BLCKSZ (for instance, WAL file > > initialization).
In those cases the cost of initializing the IOV doesn't matter, relative to the other costs. The important point is to not initialize a lot of elements if they're not even needed. Because we need to overwrite the trailing iov element, it doesn't seem worth to try to "pre-initialize" iov. Referencing a static variable is more expensive than accessing an on-stack variable. Having a first-call check is more expensive than not having it. Thus making the iov and zbuf_sz static isn't helpful. Particularly the latter seems like a bad idea, because it's a compiler constant. > It seems to me that v3 would do extra initializations only if > pg_pwritev_with_retry() does *not* retry its writes, but that's not > the case as it retries on a partial write as per its name. The number > of iov buffers is stricly capped by remaining_size. I don't really understand this bit? Greetings, Andres Freund
