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Date: Sat, 5 Jan 2013 11:26:42 +0800 From: Fengguang Wu <fengguang.wu@...el.com> To: Simon Jeons <simon.jeons@...il.com> Cc: Namjae Jeon <linkinjeon@...il.com>, Jan Kara <jack@...e.cz>, Wanpeng Li <liwanp@...ux.vnet.ibm.com>, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Namjae Jeon <namjae.jeon@...sung.com>, Vivek Trivedi <t.vivek@...sung.com>, Dave Chinner <dchinner@...hat.com> Subject: Re: [PATCH] writeback: fix writeback cache thrashing > > > Hi Namjae, > > > > > > Why use bdi_stat_error here? What's the meaning of its comment "maximal > > > error of a stat counter"? > > Hi Simon, > > > > As you know bdi stats (BDI_RECLAIMABLE, BDI_WRITEBACK …) are kept in > > percpu counters. > > When these percpu counters are incremented/decremented simultaneously > > on multiple CPUs by small amount (individual cpu counter less than > > threshold BDI_STAT_BATCH), > > it is possible that we get approximate value (not exact value) of > > these percpu counters. > > In order, to handle these percpu counter error we have used > > bdi_stat_error. bdi_stat_error is the maximum error which can happen > > in percpu bdi stats accounting. > > > > bdi_stat(bdi, BDI_RECLAIMABLE); > > -> This will give approximate value of BDI_RECLAIMABLE by reading > > previous value of percpu count. > > > > bdi_stat_sum(bdi, BDI_RECLAIMABLE); > > ->This will give exact value of BDI_RECLAIMABLE. It will take lock > > and add current percpu count of individual CPUs. > > It is not recommended to use it frequently as it is expensive. We > > can better use “bdi_stat” and work with approx value of bdi stats. > > > > Hi Namjae, thanks for your clarify. > > But why compare error stat count to bdi_bground_thresh? What's the It's not comparing bdi_stat_error to bdi_bground_thresh, but rather, in concept, comparing bdi_stat (with error bound adjustments) to bdi_bground_thresh. > relationship between them? I also see bdi_stat_error compare to > bdi_thresh/bdi_dirty in function balance_dirty_pages. Here, it's trying to use bdi_stat_sum(), the accurate (however more costly) version of bdi_stat(), if the error would possibly be large: if (bdi_thresh < 2 * bdi_stat_error(bdi)) { bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE); //... } else { bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE); //... } Here the comment should have explained it well: * In theory 1 page is enough to keep the comsumer-producer * pipe going: the flusher cleans 1 page => the task dirties 1 * more page. However bdi_dirty has accounting errors. So use * the larger and more IO friendly bdi_stat_error. */ if (bdi_dirty <= bdi_stat_error(bdi)) break; Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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