From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: fscache recursive hang -- similar to loopback NFS issues Date: Wed, 30 Jul 2014 12:19:35 +1000 Message-ID: <20140730121935.124bc7c9@notabene.brown> References: <20140721164044.2845f3fd@notabene.brown> <29057.1406650354@warthog.procyon.org.uk> <20140730071735.21ab7ca6@notabene.brown> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; boundary="Sig_/asB.5b=FcD4nt5q5xfqNt.0"; protocol="application/pgp-signature" Cc: David Howells , ceph-devel , "linux-fsdevel@vger.kernel.org" , "linux-cachefs@redhat.com" To: Milosz Tanski Return-path: Received: from cantor2.suse.de ([195.135.220.15]:56417 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754688AbaG3CUD (ORCPT ); Tue, 29 Jul 2014 22:20:03 -0400 In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: --Sig_/asB.5b=FcD4nt5q5xfqNt.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Tue, 29 Jul 2014 21:48:34 -0400 Milosz Tanski wrote: > I would vote on the lower end of the spectrum by default (closer to > 100ms) since I imagine anybody deploying this in production > environment would likely be using SSD drives for the caching. And in > my tests on spinning disks there was little to no benefit outside of > reducing network traffic. Maybe I'm confused...... I thought the whole point of this patch was to avoid deadlocks. Now you seem to be talking about a performance benefit. What did I miss? NeilBrown >=20 > - Milosz >=20 > On Tue, Jul 29, 2014 at 5:17 PM, NeilBrown wrote: > > On Tue, 29 Jul 2014 17:12:34 +0100 David Howells = wrote: > > > >> Milosz Tanski wrote: > >> > >> > That's the same thing exact fix I started testing on Saturday. I fou= nd that > >> > there already is a wait_event_timeout (even without your recent chan= ges). The > >> > thing I'm not quite sure is what timeout it should use? > >> > >> That's probably something to make an external tuning knob for. > >> > >> David > > > > Ugg. External tuning knobs should be avoided wherever possible, and al= ways > > come with detailed instructions on how to tune them > > > > In this case I think it very nearly doesn't matter *at all* what value = is > > used. > > > > If you set it a bit too high, then on the very very rare occasion that = it > > would currently deadlock, you get a longer-than-necessary wait. So jus= t make > > sure that is short enough that by the time the sysadmin notices and sta= rts > > looking for the problem, it will be gone. > > > > And if you set it a bit too low, then it will loop around to find anoth= er > > page to deal with before that one is finished being written out, and so= maybe > > do a little bit more work than is needed (though it'll be needed eventu= ally). > > > > So the perfect number is somewhere between the typical response time for > > storage, and the typical response time for the sys-admin. Anywhere bet= ween > > 100ms and 10sec would do. 1 second is the geo-mean. > > > > (sorry I didn't reply earlier - I missed you email somehow). > > > > NeilBrown >=20 >=20 >=20 --Sig_/asB.5b=FcD4nt5q5xfqNt.0 Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (GNU/Linux) iQIVAwUBU9hWNznsnt1WYoG5AQLUlw//Zm+SXXyzheobKhZ7nuGNXWpo/+CmRLHz IUKHjFnxeptkXqOXt5uhjQhPFIFV4iOOCh1wlXp3AY5ZCUJ2NfgGLwXX6vu24uHl Owja0OagN3tDEgyGDzb8FDzZhfulV9Kgcet95skSGNk3apDt1y+Aak0OZYq5Q7WB CAA/uFjEYVXMTqIq/TqwlZtfStKuw7sOjnExWwUSzJj2WGqe5RdwGFYNrenFNdET D7Kz0yTF26jzT+73asgYOVjH1LWp58kxbdlNVs43nhv7p7327poE3MW/iN/2olOZ 8reUtYXpYBPXOsvnGdiq+z5AGWc6UT3HUvhE2nhwDm6fLAwiH4jdWa5h7qteIkFU ZZD0Sc/z7QNXYslz5mYkoDma9qPYzuDK4UDasspyxAmbOEEf/5bA7b2++vhA9Uxl IzU0w6OiYmlNwksWEgfW1TAqlxwPYUprcvuftIt+d6pzyAXcRZxPcogPDn+/02V0 S7tiPbIZNzR9Kt5uP59OSApdyzAiFGrIgEaia1sfGJd1059/XFDAg20ZUmS6l7Iv RxT2hm+2Ef5+S+SrlyFKSX2E7YG0TbhGH031YN3y1EhOLSejqV2O49nzt7m4gebW GhuvsCLBIqQXYF5SCOALzjKbNjB94lDJESLKPB7TEBERX21FAfwECKXD000W2Htl YJdw8N0n7T8= =3qjX -----END PGP SIGNATURE----- --Sig_/asB.5b=FcD4nt5q5xfqNt.0--