Date: Wed, 22 May 2019 10:47:00 -0500 From: Karl Denninger <karl@denninger.net> To: freebsd-fs@freebsd.org Subject: Re: Commit r345200 (new ARC reclamation threads) looks suspicious to me - second potential problem Message-ID: <89064e9c-251a-a065-3a72-ac65c884d51d@denninger.net> In-Reply-To: <28c7430b-fb7c-6472-5c1b-fa3ff63a9e73@FreeBSD.org> References: <369cb1e9-f36a-a558-6941-23b9b811825a@FreeBSD.org> <20190520164202.GA2130@spy> <28c7430b-fb7c-6472-5c1b-fa3ff63a9e73@FreeBSD.org>
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This is a cryptographically signed message in MIME format. --------------ms020004070907060108040100 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable On 5/22/2019 10:19 AM, Alexander Motin wrote: > On 20.05.2019 12:42, Mark Johnston wrote: >> On Mon, May 20, 2019 at 07:05:07PM +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 cache= s. >>> 4962 */ >>> 4963 arc_kmem_reap_soon(); >>> 4964 =09 >>> 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 re= aping >>> 4969 * caches, while we're near arc_c_min. Waiting here al= so >>> gives the >>> 4970 * subsequent free memory check a chance of finding tha= t the >>> 4971 * asynchronous reap has already freed enough memory, a= nd >>> we don't >>> 4972 * need to call arc_reduce_target_size(). >>> 4973 */ >>> 4974 delay((hz * arc_kmem_cache_reap_retry_ms + 999) / 1000)= ; >>> 4975 =09 >>> >>> But looks like `arc_kmem_reap_soon()` is synchronous on FreeBSD! So= , >>> this `delay()` looks very wrong. Am I right? > Why is it wrong? > >>> Looks like it should be `#ifdef illumos`. >> See also r338142, which I believe was reverted by the update. > My r345200 indeed reverted that value, but I don't see a problem there.= > When OS need more RAM, pagedaemon will drain UMA caches by itself. I > don't see a point in re-draining UMA caches in a tight loop without > delay. If caches are not sufficient to sustain one second of workload,= > then usefulness of such caches is not very clear and shrinking ARC to > free some space may be a right move. Also making ZFS drain its caches > more active then anything else in a system looks unfair to me. There is a long-lasting pathology with the older implementation. The=20 short answer is that if you have cache in UMA but unallocated to current = working set it's completely wasted -- unless quickly re-used.=C2=A0 So a = small buffer between current and allocation is ok, but the UMA system=20 will leave large amounts out but unused. Reclaiming that after a=20 reasonable amount of time is a very good thing. The other problem is that disk cache should NEVER be preferred over=20 working set space.=C2=A0 It's always wrong to do so because a working set= =20 page-out is 1 *guaranteed* I/O (to page it out) and possibly 2 I/Os (if=20 required again and thus must be recalled) while a disk cache page is 1=20 *possible* I/O avoided (if the disk cache block is requested again) It is never the right move to intentionally take an I/O in order to=20 avoid a *possible* I/O. Under certain workloads making that choice leads = to severe pathological behavior (~30 second "pauses" where the system is = doing I/O like crazy but a desired process -- such as a database, or=20 shell, does nothing waiting on working set to be paged back in) when=20 there are gigabytes (or 10s of gigabytes) of ARC outstanding. --=20 -- Karl Denninger /The Market-Ticker/ S/MIME Email accepted and preferred --------------ms020004070907060108040100 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgMFADCABgkqhkiG9w0BBwEAAKCC DdgwggagMIIEiKADAgECAhMA5EiKghDOXrvfxYxjITXYDdhIMA0GCSqGSIb3DQEBCwUAMIGL MQswCQYDVQQGEwJVUzEQMA4GA1UECAwHRmxvcmlkYTESMBAGA1UEBwwJTmljZXZpbGxlMRkw FwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRhIFN5c3RlbXMgQ0ExITAf 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