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Thu, 24 Jun 2021 09:33:18 -0700 (PDT) MIME-Version: 1.0 References: <20210624152130.877-1-sjpark@amazon.de> In-Reply-To: <20210624152130.877-1-sjpark@amazon.de> From: Shakeel Butt Date: Thu, 24 Jun 2021 09:33:07 -0700 Message-ID: Subject: Re: [PATCH v31 05/13] mm/damon: Implement primitives for the virtual memory address spaces To: SeongJae Park Cc: Jonathan.Cameron@huawei.com, acme@kernel.org, alexander.shishkin@linux.intel.com, amit@kernel.org, benh@kernel.crashing.org, Brendan Higgins , Jonathan Corbet , David Hildenbrand , dwmw@amazon.com, Marco Elver , "Du, Fan" , foersleo@amazon.de, greg@kroah.com, Greg Thelen , guoju.fgj@alibaba-inc.com, jgowans@amazon.com, Mel Gorman , mheyne@amazon.de, Minchan Kim , Ingo Molnar , namhyung@kernel.org, "Peter Zijlstra (Intel)" , Rik van Riel , David Rientjes , Steven Rostedt , Mike Rapoport , Shuah Khan , sieberf@amazon.com, snu@zelle79.org, Vlastimil Babka , Vladimir Davydov , zgf574564920@gmail.com, linux-damon@amazon.com, Linux MM , linux-doc@vger.kernel.org, LKML Content-Type: text/plain; charset="UTF-8" Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=google.com header.s=20161025 header.b=im0cOOnq; spf=pass (imf12.hostedemail.com: domain of shakeelb@google.com designates 209.85.167.49 as permitted sender) smtp.mailfrom=shakeelb@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 90481540 X-Stat-Signature: rz1uswjjb8xudikp5d9ah7ych9dto9ib X-HE-Tag: 1624552400-271113 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jun 24, 2021 at 8:21 AM SeongJae Park wrote: > > From: SeongJae Park > > On Thu, 24 Jun 2021 07:42:44 -0700 Shakeel Butt wrote: > > > On Thu, Jun 24, 2021 at 3:26 AM SeongJae Park wrote: > > > > > [...] > > > > > +/* > > > > > + * Get the three regions in the given target (task) > > > > > + * > > > > > + * Returns 0 on success, negative error code otherwise. > > > > > + */ > > > > > +static int damon_va_three_regions(struct damon_target *t, > > > > > + struct damon_addr_range regions[3]) > > > > > +{ > > > > > + struct mm_struct *mm; > > > > > + int rc; > > > > > + > > > > > + mm = damon_get_mm(t); > > > > > + if (!mm) > > > > > + return -EINVAL; > > > > > + > > > > > + mmap_read_lock(mm); > > > > > + rc = __damon_va_three_regions(mm->mmap, regions); > > > > > + mmap_read_unlock(mm); > > > > > > > > This is being called for each target every second by default. Seems > > > > too aggressive. Applications don't change their address space every > > > > second. I would recommend to default ctx->primitive_update_interval to > > > > a higher default value. > > > > > > Good point. If there are many targets and each target has a huge number of > > > VMAs, the overhead could be high. Nevertheless, I couldn't find the overhead > > > in my test setup. Also, it seems someone are already started exploring DAMON > > > patchset with the default value. and usages from others. Silently changing the > > > default value could distract such people. So, if you think it's ok, I'd like > > > to change the default value only after someone finds the overhead from their > > > usages and asks a change. > > > > > > If you disagree or you found the overhead from your usage, please feel free to > > > let me know. > > > > > > > mmap lock is a source contention in the real world workloads. We do > > observe in our fleet and many others (like Facebook) do complain on > > this issue. This is the whole motivation behind SFP, maple tree and > > many other mmap lock scalability work. I would be really careful to > > add another source of contention on mmap lock. Yes, the user can > > change this interval themselves but we should not burden them with > > this internal knowledge like "oh if you observe high mmap contention > > you may want to increase this specific interval". We should set a good > > default value to avoid such situations (most of the time). > > Thank you for this nice clarification. I can understand your concern because I > also worked for an HTM-based solution of the scalability issue before. > > However, I have neither strong preference nor confidence for the new default > value at the moment. Could you please recommend one if you have? > I would say go with a conservative value like 60 seconds. Though there is no scientific reason behind this specific number, I think it would be a good compromise. Applications usually don't change their address space layout that often.