On Sat, 2003-08-02 at 03:49, Nick Piggin wrote: > Ian Kumlien wrote: > > >On Fri, 2003-08-01 at 08:27, Nick Piggin wrote: > > > >>To start with its CFQ. Also could you clarify what you mean by > >>load and what you mean by CFQ doing nothing, and why AS is overhead > >>in the no load case. I can't really follow what you are saying. > >CFQ passes the req's on directly until there is enough load... In the > >load case it builds queues. Just like SFQ (but sfq can drop packets > >afair). > > > What do you mean? CFQ merges and sorts requests, and it services > each process in a round robin manner. I don't have the CFQ code > at hand, but I don't think it does anything different in the > "load" case. Yes, i thought it worked in a different manner, as Jens Axboe pointed out. I was under the impression that it worked differently. > >This way, we wouldn't have the initial > >'can-we-merge-this-with-other-data-coming' delay when not needed. > No that is what queue plugging can be used for. Queue plugging? > >If as could be attached to the 'queue build up' then AS would only be > >doing what it's good at, throughput and minimizing head movements. > No, AS does only try to do what it is good at. As complex as it > is, its meant to be almost as simple as possible. Oh? from what i've gathered from my somewhat on and off readings of lkml i thought that 1, AS added a delay to gather additional requests prior to sending the request to the disk. 2, (as i found out now) it also waits before moving the head. Thats what i wanted to by pass in the "hey, i'm just loading my 10 bytes config file, and nothing else is happening" scenario. > >Also patches that move prioritized data (data for processes with high > >pri) would fit right in there since you'd only be doing it during actual > >load. > I'm sorry but I'm having trouble working out what you are trying to say. > The disk gets its work in the form of a request. Linux keeps a queue of > outstanding requests for each disk. The IO schedulers are a layer between > the disk (driver) and the request queue. They get to choose the next > request that goes to the disk. AS is only set apart because it sometimes > chooses not to send a request at all even if there are some available. Which sounds odd... > Now what do you mean by disk load? And actual load? I can imagine that > if there are no requests you would say there is no load on the disk. If > there is 1 request there must be some actual load? Maybe you mean more > than 1 process with outstanding requests? Lets say that when the request queue has reached a certain length, the disk is loaded. I also have to say that i have no clue of the kernels real inner workings esp not the io layer. I have only played with QoS, and i started when it was undocumented.. =). Lets say that we could use a CBQ queue with the right bw limits. Now, lets say that when 70% of that bw is filled, we use a diff approach to treating the data... We might drop new syns since we might be paranoid about getting DoSed (stupid example, i know). Now, Translate this in to the IO part. When we have a request backlog for X entries during Y 'timeunits' we use a diff way to access the disk. And since we know that when this mode is triggered, we have a backlog of requests that we can just feed AS and thus make as do it's best. Lower latency on small things and better throughput, what could be wrong about that? [Side note: In my understanding of SFQ it does 'nothing' to data that can be sent directly, but queues when it can't (due to load)] [Also: load is just a very fuzzy term for something that might be highly complex. ie, the right value for the job in determining how the request should be handled.] -- Ian Kumlien