From mboxrd@z Thu Jan 1 00:00:00 1970 From: Fengguang Wu Subject: Re: [PATCH] writeback: fix writeback cache thrashing Date: Sat, 5 Jan 2013 15:38:46 +0800 Message-ID: <20130105073846.GA11811@localhost> References: <1356847190-7986-1-git-send-email-linkinjeon@gmail.com> <20121231113054.GC7564@quack.suse.cz> <20130102134334.GB30633@quack.suse.cz> <1357261151.5105.2.camel@kernel.cn.ibm.com> <1357346803.5273.10.camel@kernel.cn.ibm.com> <20130105032642.GA8188@localhost> <1357363603.5273.16.camel@kernel.cn.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Cc: Namjae Jeon , Jan Kara , Wanpeng Li , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Namjae Jeon , Vivek Trivedi , Dave Chinner To: Simon Jeons Return-path: Content-Disposition: inline In-Reply-To: <1357363603.5273.16.camel@kernel.cn.ibm.com> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, Jan 04, 2013 at 11:26:43PM -0600, Simon Jeons wrote: > On Sat, 2013-01-05 at 11:26 +0800, Fengguang Wu wrote: > > > > > Hi Namjae, > > > > > > > > > > Why use bdi_stat_error here? What's the meaning of its comment = "maximal > > > > > error of a stat counter"? > > > > Hi Simon, > > > >=20 > > > > As you know bdi stats (BDI_RECLAIMABLE, BDI_WRITEBACK =E2=80=A6) = are kept in > > > > percpu counters. > > > > When these percpu counters are incremented/decremented simultaneo= usly > > > > on multiple CPUs by small amount (individual cpu counter less tha= n > > > > 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 hap= pen > > > > in percpu bdi stats accounting. > > > >=20 > > > > bdi_stat(bdi, BDI_RECLAIMABLE); > > > > -> This will give approximate value of BDI_RECLAIMABLE by readin= g > > > > previous value of percpu count. > > > >=20 > > > > bdi_stat_sum(bdi, BDI_RECLAIMABLE); > > > > ->This will give exact value of BDI_RECLAIMABLE. It will take lo= ck > > > > and add current percpu count of individual CPUs. > > > > It is not recommended to use it frequently as it is expensive.= We > > > > can better use =E2=80=9Cbdi_stat=E2=80=9D and work with approx va= lue of bdi stats. > > > >=20 > > >=20 > > > Hi Namjae, thanks for your clarify. > > >=20 > > > But why compare error stat count to bdi_bground_thresh? What's the > >=20 > > 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. > >=20 > > > relationship between them? I also see bdi_stat_error compare to > > > bdi_thresh/bdi_dirty in function balance_dirty_pages.=20 > >=20 >=20 > Hi Fengguang, >=20 > > 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: >=20 > Why error is large use bdi_stat_sum and error is few use bdi_stat? It's the opposite. Please check this per-cpu counter routine to get an id= ea: /* * Add up all the per-cpu counts, return the result. This is a more accu= rate * but much slower version of percpu_counter_read_positive() */ =20 s64 __percpu_counter_sum(struct percpu_counter *fbc) > >=20 > > if (bdi_thresh < 2 * bdi_stat_error(bdi)) { > > bdi_reclaimable =3D bdi_stat_sum(bdi, BDI_REC= LAIMABLE); > > //... > > } else { > > bdi_reclaimable =3D bdi_stat(bdi, BDI_RECLAIM= ABLE); > > //... > > } > >=20 > > Here the comment should have explained it well: > >=20 > > * In theory 1 page is enough to keep the comsumer-pr= oducer > > * pipe going: the flusher cleans 1 page =3D> the tas= k dirties 1 > > * more page. However bdi_dirty has accounting errors= . So use >=20 > Why bdi_dirty has accounting errors? Because it typically uses bdi_stat() to get the rough sum of the per-cpu counters. =20 Thanks, Fengguang > > * the larger and more IO friendly bdi_stat_error. > > */ > > if (bdi_dirty <=3D bdi_stat_error(bdi)) > > break; > >=20 > >=20 > > Thanks, > > Fengguang >=20 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org