On 4/21/18 8:07 AM, Zhengyuan Liu wrote: > 2018-04-20 22:34 GMT+08:00 Jens Axboe <axboe@xxxxxxxxx>: >> On 4/19/18 9:51 PM, Zhengyuan Liu wrote: >>> Hi, Shaohua >>> >>> I found it indeed doesn't do front merge when two threads flush plug list concurrently. To >>> reappear , I prepared two IO threads , named a0.io and a1.io . >>> Thread a1.io uses libaio to write 5 requests : >>> sectors: 16 + 8, 40 + 8, 64 + 8, 88 + 8, 112 + 8 >>> Thread a0.io uses libaio to write other 5 requests : >>> sectors: 8+ 8, 32 + 8, 56 + 8, 80 + 8, 104 + 8 >> >> I'm cutting some of the below. >> >> Thanks for the detailed email. It's mostly on purpose that we don't >> spend cycles and memory on maintaining a separate front merge hash, >> since it's generally not something that happens very often. If you have >> a thread pool doing IO and split sequential IO such that you would >> benefit a lot from front merging, then I would generally claim that >> you're not writing your app in the most optimal manner. >> > > Thanks for explanation, I only consider the problem through the code's > perspective and ignore the reality situation of app. That's quite by design and not accidental. >> So I'm curious, what's the big interest in front merging? > > If it's not something that happens so much often, I think it's not worth to > support front merging too. > > By the way, I got another question that why not blktrace tracing the back > merging of requests while flushing plugged requests to queue, if it does > we may get a more clear view about IO merging. Not sure I follow, exactly where is a back merge trace missing? -- Jens Axboe
