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[2603:7000:c01:2716:da5e:d3ff:fee7:26e7]) by smtp.gmail.com with ESMTPSA id vy22-20020a05620a491600b0078597896394sm31677qkn.51.2024.02.28.13.44.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Feb 2024 13:44:21 -0800 (PST) Date: Wed, 28 Feb 2024 16:44:20 -0500 From: Johannes Weiner To: Byungchul Park Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com Subject: Re: [PATCH] mm/vmscan: simplify the calculation of fractions for SCAN_FRACT Message-ID: <20240228214420.GA50163@cmpxchg.org> References: <20240228015500.52452-1-byungchul@sk.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240228015500.52452-1-byungchul@sk.com> X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 906B5180017 X-Stat-Signature: tknocgexaz33dcitk7qhtreh8amsducn X-Rspam-User: X-HE-Tag: 1709156663-858177 X-HE-Meta: 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 R2WVZq4m 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.002502, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Feb 28, 2024 at 10:55:00AM +0900, Byungchul Park wrote: > The current way to calculate fractions for SACN_FRACT is little readable > and more complicated than it should be. It also performs unnecessary > division and adjustment to avoid zero operands. Prune away by > multiplying the fractions by 'anon_cost * file_cost / (3 * total_cost)': > > where: > total_cost = sc->anon_cost + sc->file_cost > anon_cost = total_cost + sc->anon_cost > file_cost = total_cost + sc->file_cost > > before: > fraction[0] = swappiness * 3 * total_cost / anon_cost > fraction[1] = (200 - swappiness) * 3 * total_cost / file_cost > > after: > fraction[0] = swappiness * file_cost > fraction[1] = (200 - swappiness) * anon_cost > > Worth noting that this patch doesn't change the formula. > > Signed-off-by: Byungchul Park > --- > mm/vmscan.c | 14 +++----------- > 1 file changed, 3 insertions(+), 11 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 4657440854db..7b33fcc1cbdc 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -2339,7 +2339,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc, > u64 fraction[ANON_AND_FILE]; > u64 denominator = 0; /* gcc */ > enum scan_balance scan_balance; > - unsigned long ap, fp; > enum lru_list lru; > > /* > @@ -2416,17 +2415,10 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc, > total_cost = sc->anon_cost + sc->file_cost; > anon_cost = total_cost + sc->anon_cost; > file_cost = total_cost + sc->file_cost; > - total_cost = anon_cost + file_cost; > > - ap = swappiness * (total_cost + 1); > - ap /= anon_cost + 1; > - > - fp = (200 - swappiness) * (total_cost + 1); > - fp /= file_cost + 1; > - > - fraction[0] = ap; > - fraction[1] = fp; > - denominator = ap + fp; > + fraction[0] = swappiness * file_cost; > + fraction[1] = (200 - swappiness) * anon_cost; Unfortunately, I don't think that anon = swappiness * file_cost file = (200 - swappiness) * anon_cost is more readable. Sure it's the same, but I think it's clearer to actually see that `anon = total_cost / anon_cost` ratio in the code.