Re: performance: use pread instead of lseek+read

Manfred Spraul wrote:
> Bruce Momjian wrote:
>
> >BSD/OS doesn't have pread either. Isn't pread() just a case of merging
> >two system calls into one? Does a single system call cause that much
> >overhead? I didn't think so.
> >
> >
> As I wrote, in a microbenchmark lseek+read(,8192) was 10% slower than
> pread(,,8192).
>
> >Doesn't pread() do the lseek() internally anyway.
> >
> No. pread doesn't use the file pointer at all.
> This is a huge advantage if fds are shared: Two threads/processes can
> read simultaneously from the same fd. This is impossible without pread -
> there is only one file pointer, the threads would trash each others state.
>
> Since postgresql doesn't share fds, the only advantage for postgresql is
> the lower syscall overhead.

Yes, I can imaging having file descriptors shared like that would be a
big win, and I guess that's why it is called pread (pthread). Anyway,
for us, it does seem like just a merged lseek/read() call, and because
we can avoid the lseek() sometimes, I wonder if our code may be faster
sometimes. I can also imagine the separate lseek()/read() calls to be
better optimized by the kernel because a read with out an lseek is more
clearly sequential.

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