would be of making the low watermark so close to the high watermark. It would mean that the disk would be read more often, but how severely would it impact courier's performance?
Depends on the disk subsystem.
Has anyone else experimented with these values? And if so, what is your experience?
Sam, do you have anything to add to this discussion based on your personal testing and experience? I am curious as to how you devised this algorithm. Is it a well-known algorithm, with documented tuning guidelines, or is it something you have devised yourself?
It's my own invention. It's not much of an algorithm, really. The goal is to avoid hitting the disk after every delivery attempt. That's it.
When you have a large backup, something has to give. Perfect FIFO scheduling either requires some sort of a disk-based database structure, that orders messages by the next scheduled delivery attempt date, or to keep everything in memory.
The current algorithm avoids either unwanted alternative. The compromise is that if the queue is dominated by a single domain not all available delivery slots will be used. But as long as you've maxed out your network bandwidth the additional pending deliveries won't get you anything anyway. So all you need to make sure is that you're using all of your available bandwidth.
The other thing you've noticed is new, incoming mail, starving out the backed up queue. This might be fixable by simply not queuing up new mail while the in-memory queue is above the low watermark. Just queue it to disk. When the in-memory queue hits the low watermark it'll just go to the disk anyway.
But, the flip side of the coin is that some may want to give preferential treatment to their primary destination mail domain when there's a large queue backup. This might even be a tunable setting.
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