Re: excessive number of db writer

Hm after 13 messages no one actually pointed out the side effects? :-P

On 9/6/06, Kevin Closson <kevinc_at_polyserve.com> wrote:
>
> >>>
> >>>
> >>>Is it known what exactly is the difference between Oracle's
> >>>dbwr "master-slaves" and "mutiple dbwr"? As far as I
> >>>understand it is about making access to LRU *parallel* by
> >>>adding multiply LRU lists, isn't it?
>
> the difference lies entirely in the fact that writer slaves
> do nothing more than perform writes. They do not grab latches.
> That is, writer slaves do not grab latches.
>
> It is a poor-man's way to get quasi-asynchronous buffer flushing,
> but it actually stinks a bit because the freeing activity by the
> master is whole-parcel activity. So, if the master hands, say, 256
> writes to each of two slaves, the master does not free the 512
> buffers until ALL the 256 writes from each writer have completed
> and the slaves posted back to the master. If the slaves use
> synchronous I/O... uh, you know what I'm going to say ...
>
> If they use async I/O, a sub-batch is only as fast as the slowest
> write in the sub-batch. So, if one of the I/Os in one of the
> 256 sub-batches happens to take 50 milliseconds, but the other
> 255 took, on average, 8, the master does not get to free any
> of those buffers for 50 milliseconds because the slave loop
> keeps him there until all are done. This model is really
> inefficient from a processor cache basis. The bus traces always
> look like a fresh road-kill when compared to single process
> async I/O dbwr... especially when compined with an elegant
> venctored semop so that a single system call can post
> all the busy buffer waiters ... ahhh...port optimizations...
> the good old days ... gone with the wind (thanks linux).
>
>
>
>
>
>
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> >>>http://www.landsbanki.is/disclaimer
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