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[2003:cb:c704:4900:849b:f76e:5e1f:ff95]) by smtp.gmail.com with ESMTPSA id r9-20020a05600c35c900b0038c7626c454sm12461150wmq.12.2022.03.21.08.45.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Mar 2022 08:45:41 -0700 (PDT) Message-ID: <0414c610-7f56-2dd2-0d83-ac3a5194eb60@redhat.com> Date: Mon, 21 Mar 2022 16:45:40 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Content-Language: en-US To: CGEL Cc: bsingharora@gmail.com, akpm@linux-foundation.org, yang.yang29@zte.com.cn, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20220316133420.2131707-1-yang.yang29@zte.com.cn> <412dc01c-8829-eac2-52c7-3f704dbb5a98@redhat.com> <6232970f.1c69fb81.4e365.c9f2@mx.google.com> <4e76476b-1da0-09c5-7dc4-0b2db796a549@redhat.com> <62330402.1c69fb81.d2ba6.0538@mx.google.com> <987bd014-c5ab-52cb-627e-2085560cb327@redhat.com> <6233e342.1c69fb81.692f.6286@mx.google.com> <2bb1c357-5335-9d96-d862-bd51c1014193@redhat.com> <6236c600.1c69fb81.7cd4.a900@mx.google.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH] delayacct: track delays from ksm cow In-Reply-To: <6236c600.1c69fb81.7cd4.a900@mx.google.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20.03.22 07:13, CGEL wrote: > On Fri, Mar 18, 2022 at 09:24:44AM +0100, David Hildenbrand wrote: >> On 18.03.22 02:41, CGEL wrote: >>> On Thu, Mar 17, 2022 at 11:05:22AM +0100, David Hildenbrand wrote: >>>> On 17.03.22 10:48, CGEL wrote: >>>>> On Thu, Mar 17, 2022 at 09:17:13AM +0100, David Hildenbrand wrote: >>>>>> On 17.03.22 03:03, CGEL wrote: >>>>>>> On Wed, Mar 16, 2022 at 03:56:23PM +0100, David Hildenbrand wrote: >>>>>>>> On 16.03.22 14:34, cgel.zte@gmail.com wrote: >>>>>>>>> From: Yang Yang >>>>>>>>> >>>>>>>>> Delay accounting does not track the delay of ksm cow. When tasks >>>>>>>>> have many ksm pages, it may spend a amount of time waiting for ksm >>>>>>>>> cow. >>>>>>>>> >>>>>>>>> To get the impact of tasks in ksm cow, measure the delay when ksm >>>>>>>>> cow happens. This could help users to decide whether to user ksm >>>>>>>>> or not. >>>>>>>>> >>>>>>>>> Also update tools/accounting/getdelays.c: >>>>>>>>> >>>>>>>>> / # ./getdelays -dl -p 231 >>>>>>>>> print delayacct stats ON >>>>>>>>> listen forever >>>>>>>>> PID 231 >>>>>>>>> >>>>>>>>> CPU count real total virtual total delay total delay average >>>>>>>>> 6247 1859000000 2154070021 1674255063 0.268ms >>>>>>>>> IO count delay total delay average >>>>>>>>> 0 0 0ms >>>>>>>>> SWAP count delay total delay average >>>>>>>>> 0 0 0ms >>>>>>>>> RECLAIM count delay total delay average >>>>>>>>> 0 0 0ms >>>>>>>>> THRASHING count delay total delay average >>>>>>>>> 0 0 0ms >>>>>>>>> KSM count delay total delay average >>>>>>>>> 3635 271567604 0ms >>>>>>>>> >>>>>>>> >>>>>>>> TBH I'm not sure how particularly helpful this is and if we want this. >>>>>>>> >>>>>>> Thanks for replying. >>>>>>> >>>>>>> Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want >>>>>>> save memory, it's a tradeoff by suffering delay on ksm cow. Users can >>>>>>> get to know how much memory ksm saved by reading >>>>>>> /sys/kernel/mm/ksm/pages_sharing, but they don't know what the costs of >>>>>>> ksm cow delay, and this is important of some delay sensitive tasks. If >>>>>>> users know both saved memory and ksm cow delay, they could better use >>>>>>> madvise(, , MADV_MERGEABLE). >>>>>> >>>>>> But that happens after the effects, no? >>>>>> >>>>>> IOW a user already called madvise(, , MADV_MERGEABLE) and then gets the >>>>>> results. >>>>>> >>>>> Image user are developing or porting their applications on experiment >>>>> machine, they could takes those benchmark as feedback to adjust whether >>>>> to use madvise(, , MADV_MERGEABLE) or it's range. >>>> >>>> And why can't they run it with and without and observe performance using >>>> existing metrics (or even application-specific metrics?)? >>>> >>>> >>> I think the reason why we need this patch, is just like why we need >>> swap,reclaim,thrashing getdelay information. When system is complex, >>> it's hard to precise tell which kernel activity impact the observe >>> performance or application-specific metrics, preempt? cgroup throttle? >>> swap? reclaim? IO? >>> >>> So if we could get the factor's precise impact data, when we are tunning >>> the factor(for this patch it's ksm), it's more efficient. >>> >> >> I'm not convinced that we want to make or write-fault handler more >> complicated for such a corner case with an unclear, eventual use case. > > IIRC, KSM is designed for VM. But recently we found KSM works well for > system with many containers(save about 10%~20% of total memroy), and > container technology is more popular today, so KSM may be used more. > > To reduce the impact for write-fault handler, we may write a new function > with ifdef CONFIG_KSM inside to do this job? Maybe we just want to catch the impact of the write-fault handler when copying more generally? > >> IIRC, whenever using KSM you're already agreeing to eventually pay a >> performance price, and the price heavily depends on other factors in the >> system. Simply looking at the number of write-faults might already give >> an indication what changed with KSM being enabled. >> > While saying "you're already agreeing to pay a performance price", I think > this is the shortcoming of KSM that putting off it being used more widely. > It's not easy for user/app to decide how to use madvise(, ,MADV_MERGEABLE). ... and my point is that the metric you're introducing might absolutely not be expressive for such users playing with MADV_MERGEABLE. IMHO people will look at actual application performance to figure out what "harm" will be done, no? But I do see value in capturing how many COW we have in general -- either via a counter or via a delay as proposed by you. > > Is there a more easy way to use KSM, enjoying memory saving while minimum > the performance price for container? We think it's possible, and are working > for a new patch: provide a knob for cgroup to enable/disable KSM for all tasks > in this cgroup, so if your container is delay sensitive just leave it, and if > not you can easy to enable KSM without modify app code. > > Before using the new knob, user might want to know the precise impact of KSM. > I think write-faults is indirection. If indirection is good enough, why we need > taskstats and PSI? By the way, getdelays support container statistics. Would anything speak against making this more generic and capturing the delay for any COW, not just for KSM? -- Thanks, David / dhildenb