Date: Mon, 20 May 2019 19:40:14 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Ian Lepore <ian@freebsd.org> Cc: lev@FreeBSD.org, freebsd-fs@freebsd.org, freebsd-hackers@FreeBSD.org, Alexander Motin <mav@FreeBSD.org> Subject: Re: Commit r345200 (new ARC reclamation threads) looks suspicious to me - second potential problem Message-ID: <20190520164014.GA47119@zxy.spb.ru> In-Reply-To: <8160c9149c04d2b622292abf582bcbb9a541d2ed.camel@freebsd.org> References: <369cb1e9-f36a-a558-6941-23b9b811825a@FreeBSD.org> <8160c9149c04d2b622292abf582bcbb9a541d2ed.camel@freebsd.org>
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On Mon, May 20, 2019 at 10:20:45AM -0600, Ian Lepore wrote: > On Mon, 2019-05-20 at 19:05 +0300, Lev Serebryakov wrote: > > I'm looking at last commit to > > 'sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c' (r345200) and > > have another question. > > > > Here are such code: > > > > 4960 /* > > 4961 * Kick off asynchronous kmem_reap()'s of all our > > caches. > > 4962 */ > > 4963 arc_kmem_reap_soon(); > > 4964 > > 4965 /* > > 4966 * Wait at least arc_kmem_cache_reap_retry_ms between > > 4967 * arc_kmem_reap_soon() calls. Without this check it is > > possible to > > 4968 * end up in a situation where we spend lots of time > > reaping > > 4969 * caches, while we're near arc_c_min. Waiting here > > also > > gives the > > 4970 * subsequent free memory check a chance of finding > > that the > > 4971 * asynchronous reap has already freed enough memory, > > and > > we don't > > 4972 * need to call arc_reduce_target_size(). > > 4973 */ > > 4974 delay((hz * arc_kmem_cache_reap_retry_ms + 999) / > > 1000); > > 4975 > > > > But looks like `arc_kmem_reap_soon()` is synchronous on FreeBSD! So, > > this `delay()` looks very wrong. Am I right? > > > > Looks like it should be `#ifdef illumos`. > > > > One of the things arc_kmem_reap_soon() does is call > dnlc_reduce_cache(), and that sets a variable and does a condition > variable broadcast, presumably causing other threads to wake up and do > some work. So, presumably the delay (which appears to really be a call > to pause(9) on freebsd) allows time for that async work to happen > before calling arc_available_memory(). This call perform before any kmem reap and only conditional by arc_meta_used>=arc_meta_limit. In any way kmem reap is very long operation, longest any work in arc_dnlc_evicts_thread() (vnlru_free()).
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