From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Chris Storah <cstorah(at)emis-support(dot)demon(dot)co(dot)uk> |
Cc: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: low priority postmaster threads? |
Date: | 2001-02-21 23:10:03 |
Message-ID: | 12895.982797003@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
I wrote:
> Chris Storah <cstorah(at)emis-support(dot)demon(dot)co(dot)uk> writes:
>> What I am looking for is a postgres system that runs 100 users or so at
>> 'full speed', and major day long queries at a 'when idle' priority.
> The trouble here is that CPU nice doesn't (on most platforms) change the
> behavior of the I/O scheduler, so this would only be of use to the
> extent that your queries are CPU bound and not I/O bound.
Now that I think twice, there's an even more severe problem with trying
to nice() down an individual backend, namely priority inversion.
What happens when the low-priority process holds some lock or other,
and then a higher-priority process comes along and wants the lock?
The high-priority process has to wait, that's what. But there's no
mechanism to raise the priority of the lower-priority lock holder, which
means that the high-priority process is now effectively lowered to the
lower priority; it may have to wait quite a long time, if there are
other high-priority processes sucking CPU away from the low-priority
guy.
In short, forget about nice'ing an individual backend; you probably
won't like the results. Sorry.
regards, tom lane
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