Comment 1 for bug 1804453

This proved to be a significant issue while working on https://review.openstack.org/#/c/619248/ , a performance measuring job that uses placeload. That uses aiohttp to make concurrent connections to placement. At high concurrency _ensure_aggregate loops a great deal and causes the server to block enough that the client starts experiencing errors because it cannot make a good connection.

I fixed it by preheating the aggregates so that _ensure_aggregate almost always returns after getting the aggregate id, rather than looping to try to create it. With that in place things are very smooth.

That experience suggests we should fix this, because it seems likely that operators might like to do mass aggregate management and use asyncio-based tools to do it, or maybe something from languages like go where similar behaviour might happen.