Re: Experimental patch for inter-page delay in VACUUM

Bruce Momjian wrote:

> Jan Wieck wrote:
>> Bruce Momjian wrote:
>> > I would be interested to know if you have the background write process
>> > writing old dirty buffers to kernel buffers continually if the sync()
>> > load is diminished.  What this does is to push more dirty buffers into
>> > the kernel cache in hopes the OS will write those buffers on its own
>> > before the checkpoint does its write/sync work.  This might allow us to
>> > reduce sync() load while preventing the need for O_SYNC/fsync().
>> 
>> I tried that first. Linux 2.4 does not, as long as you don't tell it by 
>> reducing the dirty data block aging time with update(8). So you have to 
>> force it to utilize the write bandwidth in the meantime. For that you 
>> have to call sync() or fsync() on something.
>> 
>> Maybe O_SYNC is not as bad an option as it seems. In my patch, the 
>> checkpointer flushes the buffers in LRU order, meaning it flushes the 
>> least recently used ones first. This has the side effect that buffers 
>> returned for replacement (on a cache miss, when the backend needs to 
>> read the block) are most likely to be flushed/clean. So it reduces the 
>> write load of backends and thus the probability that a backend is ever 
>> blocked waiting on an O_SYNC'd write().
>> 
>> I will add some counters and gather some statistics how often the 
>> backend in comparision to the checkpointer calls write().
> 
> OK, new idea.  How about if you write() the buffers, mark them as clean
> and unlock them, then issue fsync().  The advantage here is that we can

Not really new, I think in my first mail I wrote that I simplified this 
new mdfsyncrecent() function by calling sync() instead ... other than 
that the code I posted worked exactly that way.


Jan

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