Re: Experimental patch for inter-page delay in VACUUM

Shridhar Daithankar wrote:
> On Tuesday 11 November 2003 00:50, Neil Conway wrote:
>> Jan Wieck <JanWieck@Yahoo.com> writes:
>> > We can't resize shared memory because we allocate the whole thing in
>> > one big hump - which causes the shmmax problem BTW. If we allocate
>> > that in chunks of multiple blocks, we only have to give it a total
>> > maximum size to get the hash tables and other stuff right from the
>> > beginning. But the vast majority of memory, the buffers themself, can
>> > be made adjustable at runtime.
>>
>> Yeah, writing a palloc()-style wrapper over shm has been suggested
>> before (by myself among others). You could do the shm allocation in
>> fixed-size blocks (say, 1 MB each), and then do our own memory
>> management to allocate and release smaller chunks of shm when
>> requested. I'm not sure what it really buys us, though: sure, we can
>> expand the shared buffer area to some degree, but
> 
> Thinking of it, it can be put as follows. Postgresql needs shared memory 
> between all the backends. 
> 
> If the parent postmaster mmaps anonymous memory segments and shares them with 
> children, postgresql wouldn't be dependent upon any kernel resourse aka 
> shared memory anymore.

And how does a newly mmap'ed segment propagate into a running backend?


Jan

> 
> Furthermore parent posmaster can allocate different anonymous mappings for 
> different databases. In addition to postgresql buffer manager overhaul, this 
> would make things lot better.
> 
> note that I am not suggesting mmap to maintain files on disk. So I guess that 
> should be OK. 
> 
> I tried searching for mmap on hackers. The threads seem to be very old. One in 
> 1998. with so many proposals of rewriting core stuff, does this have any 
> chance?
> 
>  Just a thought.
> 
>  Shridhar
> 
> 
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