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Wed, 22 May 2019 09:07:30 -0700 (PDT) Received: from mavoffice.ixsystems.com ([12.189.233.129]) by smtp.gmail.com with ESMTPSA id x5sm6533838ywb.50.2019.05.22.09.07.29 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 22 May 2019 09:07:30 -0700 (PDT) Sender: Alexander Motin Subject: Re: Commit r345200 (new ARC reclamation threads) looks suspicious to me - second potential problem To: lev@FreeBSD.org, Mark Johnston Cc: freebsd-fs@freebsd.org, freebsd-hackers@freebsd.org References: <369cb1e9-f36a-a558-6941-23b9b811825a@FreeBSD.org> <20190520164202.GA2130@spy> <28c7430b-fb7c-6472-5c1b-fa3ff63a9e73@FreeBSD.org> <94d051a3-3427-7a5b-efe7-169cff2265d3@FreeBSD.org> From: Alexander Motin Openpgp: preference=signencrypt Autocrypt: addr=mav@FreeBSD.org; prefer-encrypt=mutual; keydata= xsBNBFOzxAwBCADkPrax0pI2W/ig0CK9nRJJwsHitAGEZ2HZiFEuti+6/4UVxj81yr4ak/4g 9bKUyC7rMEAp/ZHNhd+MFCPAAcHPvtovnfykqE/vuosCS3wlSLloix2iKVLks0CwbLHGAyne 46lTQW74Xl/33c3W1Z6d8jD9gVFT/xaVzZ0U9xdzOmsYAZaAj4ki0tuxO9F7L+ct9grRe7iP g8t9hai7BL4ee3VRwk2JXnKb7UvBiVITKYWKz1jRvZIrjPokgEcCLOSlv7x/1kjuFnj3xWZU 7HSFFT8J93epBbrSSCsYsppIk2fZH41kaaFXsMQfTPH8wkeM6qwrvOh4HiQM08R+9tThABEB AAHNIUFsZXhhbmRlciBNb3RpbiA8bWF2QEZyZWVCU0Qub3JnPsLAlwQTAQoAQQIbAwULCQgH AwUVCgkICwUWAwIBAAIeAQIXgAIZARYhBOmM88TmnMPNDledVYMYw5VbqyJ/BQJZYMKuBQkN McyiAAoJEIMYw5VbqyJ/tuUIAOG3ONOSNYqjK4eTZ1TVh9jdUBAhWk5nhDFnODN49Wj0AbYm 7aIqy8O1hnCDSZG5LttjSAo3UfXJZDKQM0BLb0gpRMBnAYqO6tdolLNqAbPGJBnGoPjsh24y 6KcbDaNnis+lD4GwPXwQM+92wZGhCUFElPV9NciZGVS65TNIgk7X+yEjjhD1MSWKKijZ1r9Z zIt4OzUTxxNOvzdlABZS88nNRdJkatOQJPmFdd1mpP6UzTNCiLUo1pIqOEtJgvVVDYq5WHY6 tciWWYdmZG/tIBexJmv2mV2OLVjXR6ZeKmntVH14H72/wRHJuYHQC+r5SVRcWWayrThsY6jZ Yr4+raTOwE0EU7PEDAEIAOZgWf2cJIu+58IzP2dkXE/urj3tr4OqrB/yHGWUf71Lz6D0Fi6Z AXgDtmcFLGPfMyWuLAvSM+xmoguk7zC4hRBYvQycmIhuqBq1jO1Wp/Z+lpoPM/1cDYLn8Flv mI/c40MhUZh345DA4jYWWaZNjQHUWVQ1fPf595vdVVMPT/abE8E5DaF6fSkRmqFTmfYRkfbt 3ytU8NdUapDcJVY7cEP2nJBVNZPnOIObR/ZIgSxjjrG5o34yXoqeup8JvwEv+/NylzzuyXEZ R1EdEIzQ/a1nh/0j4NXtzZEqKW4aTWlmSqb6wN8jh1OSOOqkYsfnE3nfxcZbxi4IRoNQYlm5 9R8AEQEAAcLAZQQYAQoADwUCU7PEDAIbDAUJBaOagAAKCRCDGMOVW6sif7FRB/4k9y/GaGqU fcJiXdQHRAKHCUvbKMFgeEDHOg33qx+POS2Ah85/PXVa2jYBldCZDmYc+zl48aEMd163a7s3 0gJaB7CYElwxlKUk6c+5gwoYIJuJJzSzW0JzSD5ch7RIRxbfxrKdsiHrUW8AeduZWzlK6VaW RmWILgLmxfLdhEVFWxbr99GSeVFZaZwn6tl/8CvBcgYoARvJvl0V5zS1akQfEISYkwL9EfUI W44EOHranL5qUXkedXBYp6fRsooGrIimfwYxaC8FbXhk3FMgMjDMRiVq4POHo1iGeYETsUrL NM6184E25gPVtX2fb3RhM8Xh6BkwCZ6ZYbQ+AcD4F/cKwsB8BBgBCgAmAhsMFiEE6YzzxOac w80OV51VgxjDlVurIn8FAllgwtgFCQ0xzMwACgkQgxjDlVurIn9OqAf9FAcKWS95wTTbraXA qg/+bQyHgjlMtGCgkmfxLsbUGeqiFgmSIuoDrF7q6sYPs6p00CXXZRuuNZt0lX7O95re8mgz gxm5iJisZpdbHMVepYlw/AxT2wCHwxGCEe64Lm+A9vjlOd+3D3/6fSLwZ9WFCE6p6lQZ1CDg 09xe+JKSgC+KDqmn0tzGKyfSCuhRAq3XkZyxL1hxBaDeP0eeKlzoy7jXodf3wVvXXc0cmpza B5McuRHK4EU6jIioHo30YqPM4AjPHGxV2X1N6/Aayungzj9EXNZtKCxs6dsTvjniWa5VkZ9F 4SOdSbxEen1DZRYpeWnd7GVmO86n+5USkKCXPg== Message-ID: <2a50e192-e672-7c87-178b-afd509a765df@FreeBSD.org> Date: Wed, 22 May 2019 12:07:29 -0400 User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <94d051a3-3427-7a5b-efe7-169cff2265d3@FreeBSD.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 91BA48CD62 X-Spamd-Bar: ------ Authentication-Results: mx1.freebsd.org; dkim=pass header.d=gmail.com header.s=20161025 header.b=uWxXQ2qi; spf=pass (mx1.freebsd.org: domain of mavbsd@gmail.com designates 2607:f8b0:4864:20::b29 as permitted sender) smtp.mailfrom=mavbsd@gmail.com X-Spamd-Result: default: False [-6.07 / 15.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; R_DKIM_ALLOW(-0.20)[gmail.com:s=20161025]; NEURAL_HAM_MEDIUM(-1.00)[-1.000,0]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[4]; R_SPF_ALLOW(-0.20)[+ip6:2607:f8b0:4000::/36]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.10)[text/plain]; RCVD_TLS_LAST(0.00)[]; DMARC_NA(0.00)[FreeBSD.org]; TO_DN_SOME(0.00)[]; NEURAL_HAM_LONG(-1.00)[-1.000,0]; RCVD_COUNT_THREE(0.00)[3]; IP_SCORE(-2.97)[ip: (-9.28), ipnet: 2607:f8b0::/32(-3.26), asn: 15169(-2.27), country: US(-0.06)]; DKIM_TRACE(0.00)[gmail.com:+]; MX_GOOD(-0.01)[cached: alt3.gmail-smtp-in.l.google.com]; RCVD_IN_DNSWL_NONE(0.00)[9.2.b.0.0.0.0.0.0.0.0.0.0.0.0.0.0.2.0.0.4.6.8.4.0.b.8.f.7.0.6.2.list.dnswl.org : 127.0.5.0]; NEURAL_HAM_SHORT(-0.89)[-0.888,0]; FORGED_SENDER(0.30)[mav@FreeBSD.org,mavbsd@gmail.com]; MIME_TRACE(0.00)[0:+]; FREEMAIL_ENVFROM(0.00)[gmail.com]; ASN(0.00)[asn:15169, ipnet:2607:f8b0::/32, country:US]; FROM_NEQ_ENVFROM(0.00)[mav@FreeBSD.org,mavbsd@gmail.com]; MID_RHS_MATCH_FROM(0.00)[] X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 22 May 2019 16:07:33 -0000 On 22.05.2019 11:50, Lev Serebryakov wrote: > On 22.05.2019 18:19, Alexander Motin wrote: > >>>> But looks like `arc_kmem_reap_soon()` is synchronous on FreeBSD! So, >>>> this `delay()` looks very wrong. Am I right? >> >> Why is it wrong? > One second pause after synchronous operation to wait it completion? No. To rate-throttle them. This gives UMA a second to get back into minimally steady state after we ripped all caches from it. As I have told, we do not want to drain caches constantly in a tight loop, we want more or less steady state. > There one more questionable piece of code: > > 6936 static void > 6937 arc_lowmem(void *arg __unused, int howto __unused) > 6938 { > .... > 6947 arc_reduce_target_size(to_free); > 6948 > 6949 mutex_enter(&arc_adjust_lock); > 6950 arc_adjust_needed = B_TRUE; > 6951 zthr_wakeup(arc_adjust_zthr); > > > 4587 static void > 4588 arc_reduce_target_size(int64_t to_free) > 4589 { > ... > 4612 if (asize > arc_c) { > 4613 DTRACE_PROBE2(arc__shrink_adjust, uint64_t, asize, > 4614 uint64_t, arc_c); > 4615 /* See comment in arc_adjust_cb_check() on why > lock+flag */ > 4616 mutex_enter(&arc_adjust_lock); > 4617 arc_adjust_needed = B_TRUE; > 4618 mutex_exit(&arc_adjust_lock); > 4619 zthr_wakeup(arc_adjust_zthr); > 4620 } > 4621 } > > Looks like lock/flag/wakeup sequence (which is now very cheap — mutexes > are not cheap, and this mutex could become contended in low-memory > situation) could be called twice. > > Looks like `arc_reduce_target_size()` should return boolean value and > unconditional signalling in `arc_lowmem()` should become conditional. I don't think this is not a hot path to bother about it. arc_lowmem() calls should be very rare. -- Alexander Motin