On 1/24/24 2:32 AM, Christoph Hellwig wrote: > On Tue, Jan 23, 2024 at 10:34:15AM -0700, Jens Axboe wrote: >> We do several stages of merging in the block layer - the most likely one >> to work is also the cheap one, merging direct in the per-task plug when >> IO is submitted. Getting merges outside of that is a lot less likely, >> but IO schedulers may still maintain internal data structures to >> facilitate merge lookups outside of the plug. >> >> Make mq-deadline skip expensive merge lookups if the queue lock is >> already contended. The likelihood of getting a merge here is not very >> high, hence it should not be a problem skipping the attempt in the also >> unlikely event that the queue is already contended. > > I'm curious if you tried benchmarking just removing these extra > merges entirely? We've tried removing this many years ago, and the issue is generally threadpools that do related IO and hence won't merge for those cases. It's a pretty stupid case, but I'm willing to bet we'll get regressions on rotational storage reported if we just skip it entirely. Alternatively we could make it dependent on rotational or not, but seems cleaner to me to just keep it generally enabled and just skip it if we're contended. -- Jens Axboe
