[sheepdog] [PATCH 0/8] make IO requests to wait in recovery instead of busy retrying

[Liu Yuan]

On 05/22/2012 10:51 AM, levin li wrote:

> There're many cases that the request being retried, which not wait
> but directly put the request again into the request queue to make
> it run again, it may cause CPU too busy.
> 
> In our cluster with 960 nodes, when 10 nodes leave and the there're
> heavy IO request in VM, the recovery doesn't run well because there're
> too many pending requests in the request queue which are retrying IO
> requests, and it makes CPU too busy to process the recovery requests.
> 
> And also, there's race condition in recovery which keeps nodes retrying
> to recovery a single object and make the recovery work hang there.
> 
> We should not make the request retry at the same time it fails, but we
> should put it into a queue to make it sleep until the epoch or other
> needs of this request are met, then we wake it up to make it retry.
> 


For a skim over the patch set, I think this patch set need a rethink of
list usage. It takes several lists to implement 'wait queue' conception,
on both requester and requestee nodes, which looks unnecessary to me and
code doesn't comment on why choose more than one queue. What makes them
different each other?

It is really much better if we can unify those queues into only one
queue, which we rely on to queue requests, resume requests, flush requests.

Thanks,
Yuan