| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Konstantin Knizhnik <k(dot)knizhnik(at)postgrespro(dot)ru>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Sean Chittenden <seanc(at)joyent(dot)com> |
| Subject: | Re: WAL prefetch |
| Date: | 2018-06-18 20:47:41 |
| Message-ID: | 20180618204741.yf6qw445pe6hlsqn@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2018-06-18 16:44:09 -0400, Robert Haas wrote:
> On Sat, Jun 16, 2018 at 3:41 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> The posix_fadvise approach is not perfect, no doubt about that. But it
> >> works pretty well for bitmap heap scans, and it's about 13249x better
> >> (rough estimate) than the current solution (no prefetching).
> >
> > Sure, but investing in an architecture we know might not live long also
> > has it's cost. Especially if it's not that complicated to do better.
>
> My guesses are:
>
> - Using OS prefetching is a very small patch.
> - Prefetching into shared buffers is a much bigger patch.
Why? The majority of the work is standing up a bgworker that does
prefetching (i.e. reads WAL, figures out reads not in s_b, does
prefetch). Allowing a configurable number + some synchronization between
them isn't that much more work.
> - It'll be five years before we have direct I/O.
I think we'll have lost a significant market share by then if that's the
case. Deservedly so.
Greetings,
Andres Freund
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