I know that the Civi devs have been struggling with this issue so I am not complaining or asking for it to be fixed - I just want to understand how it works, so that I can work aroud it.

I am constantly getting complaints from users that data is missing from their dashboard, and when I look in the logs it is invariably due to deadlocks in `civicrm_acl_contact_cache`. These deadlocks happen on `INSERT`. Only rarely do we get them when deleting. This is the opposite to what Eileen says here, i.e. that flushing the cache is more problematic than building: https://civicrm.org/blog/eileen/478-group-contact-cache-deadlocks-improvement

Users have reports on their dashboards that rely on smart groups for their filters. For this reason I set the smart group cache timeout to 0, so that   when, for example, someone completes an activity (and the target contact goes into a different smart group as a result) the resulting change is immediately visible. Otherwise users will worry that the change they have made has "not saved" or "not worked".

I followed Eileen;s advice (in the blog post I link to above) and turned off opportunistic caching. But I found I did not have to run the cache flush as a scheduled job either - everything seems to work without it, presumably because the cache timeout is zero. 

It has not helped with the deadlocks, though.

Is a timeout of 0 simply not supported?

I think I would be able to deal with this much better if I understood how  the ACL cache relates to the smart group cache. Does the smart group timeout affect both? When does the ACL cache get flushed / filled? Do I have control over that? If someone could explain this paragraph from Eileen's blog it would help: 

>  If your smart group cache time out is every 5 minutes and your cron
> only runs every 10 minutes your cache will be 15 minutes old most of
> the time. I would expect you would want to run the cron every minute
> (& possibly reduce your time out by a minute).

I am confused about two aspects of this:
1. She is talking about a cron job that flushes the caches, but that paragraph seems to relate to building them
2. It makes sense that the ACL cache build will depend on the smart group cache. So yes, if the smart group cache is old, and the ACL cache gets built, it will have old data in it. But that assumes that the cron setting is about the ACL cache, not the smart groups. Yet the setting that turns off the opportunistic caching pertains to smart groups, and it is that setting that she says we can replace with the cron.

How can there be a difference between when the cache is flushed and when it is built, anyway? Surely refilling the cache means first you flush it, then you fill it. They cannot be separate

And for one last extremely confusing thing: I turned off the cron job. Why is the ACL cache still trying to update?