From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f54.google.com (mail-la0-f54.google.com [209.85.215.54]) by kanga.kvack.org (Postfix) with ESMTP id 51BB96B0031 for ; Thu, 16 Jan 2014 03:51:06 -0500 (EST) Received: by mail-la0-f54.google.com with SMTP id y1so2287682lam.13 for ; Thu, 16 Jan 2014 00:51:05 -0800 (PST) Received: from relay.parallels.com (relay.parallels.com. [195.214.232.42]) by mx.google.com with ESMTPS id a4si4114591laf.83.2014.01.16.00.51.04 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 16 Jan 2014 00:51:04 -0800 (PST) Message-ID: <52D79D6B.10304@parallels.com> Date: Thu, 16 Jan 2014 12:50:51 +0400 From: Vladimir Davydov MIME-Version: 1.0 Subject: Re: [PATCH 1/5] mm: vmscan: shrink all slab objects if tight on memory References: <7d37542211678a637dc6b4d995fd6f1e89100538.1389443272.git.vdavydov@parallels.com> <20140113150502.4505f661589a4a2d30e6f11d@linux-foundation.org> <52D4E5F2.5080205@parallels.com> <20140114141453.374bd18e5290876177140085@linux-foundation.org> <52D64B27.30604@parallels.com> <20140115012541.ad302526.akpm@linux-foundation.org> <52D6AF5F.2040102@parallels.com> <20140115145327.6aae2e13a9a8bba619923ac9@linux-foundation.org> In-Reply-To: <20140115145327.6aae2e13a9a8bba619923ac9@linux-foundation.org> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, devel@openvz.org, Mel Gorman , Michal Hocko , Johannes Weiner , Rik van Riel , Dave Chinner , Glauber Costa On 01/16/2014 02:53 AM, Andrew Morton wrote: > On Wed, 15 Jan 2014 19:55:11 +0400 Vladimir Davydov wrote: > >>> We could avoid the "scan 32 then scan just 1" issue with something like >>> >>> if (total_scan > batch_size) >>> total_scan %= batch_size; >>> >>> before the loop. But I expect the effects of that will be unmeasurable >>> - on average the number of objects which are scanned in the final pass >>> of the loop will be batch_size/2, yes? That's still a decent amount. >> Let me try to summarize. We want to scan batch_size objects in one pass, >> not more (to keep latency low) and not less (to avoid cpu cache >> pollution due to too frequent calls); if the calculated value of >> nr_to_scan is less than the batch_size we should accumulate it in >> nr_deferred instead of calling ->scan() and add nr_deferred to >> nr_to_scan on the next pass, i.e. in pseudo-code: >> >> /* calculate current nr_to_scan */ >> max_pass = shrinker->count(); >> delta = max_pass * nr_user_pages_scanned / nr_user_pages; >> >> /* add nr_deferred */ >> total_scan = delta + nr_deferred; >> >> while (total_scan >= batch_size) { >> shrinker->scan(batch_size); >> total_scan -= batch_size; >> } >> >> /* save the remainder to nr_deferred */ >> nr_deferred = total_scan; >> >> That would work, but if max_pass is < batch_size, it would not scan the >> objects immediately even if prio is high (we want to scan all objects). > Yes, that's a problem. > >> For example, dropping caches would not work on the first attempt - the >> user would have to call it batch_size / max_pass times. > And we do want drop_caches to work immediately. > >> This could be >> fixed by making the code proceed to ->scan() not only if total_scan is >>> = batch_size, but also if max_pass is < batch_size and total_scan is >= >> max_pass, i.e. >> >> while (total_scan >= batch_size || >> (max_pass < batch_size && total_scan >= max_pass)) ... >> >> which is equivalent to >> >> while (total_scan >= batch_size || >> total_scan >= max_pass) ... >> >> The latter is the loop condition from the current patch, i.e. this patch >> would make the trick if shrink_slab() followed the pseudo-code above. In >> real life, it does not actually - we have to bias total_scan before the >> while loop in order to avoid dropping fs meta caches on light memory >> pressure due to a large number being built in nr_deferred: >> >> if (delta < max_pass / 4) >> total_scan = min(total_scan, max_pass / 2); > Oh, is that what's it's for. Where did you discover this gem?