From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:58774 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751117AbdDEEb3 (ORCPT ); Wed, 5 Apr 2017 00:31:29 -0400 From: NeilBrown To: Ming Lei Date: Wed, 05 Apr 2017 14:31:20 +1000 Cc: Jens Axboe , linux-block , linux-mm , LKML Subject: Re: [PATCH] loop: Add PF_LESS_THROTTLE to block/loop device thread. In-Reply-To: References: <871staffus.fsf@notabene.neil.brown.name> Message-ID: <87zifvh3vr.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Sender: linux-block-owner@vger.kernel.org List-Id: linux-block@vger.kernel.org --=-=-= Content-Type: text/plain On Tue, Apr 04 2017, Ming Lei wrote: > On Mon, Apr 3, 2017 at 9:18 AM, NeilBrown wrote: >> >> When a filesystem is mounted from a loop device, writes are >> throttled by balance_dirty_pages() twice: once when writing >> to the filesystem and once when the loop_handle_cmd() writes >> to the backing file. This double-throttling can trigger >> positive feedback loops that create significant delays. The >> throttling at the lower level is seen by the upper level as >> a slow device, so it throttles extra hard. >> >> The PF_LESS_THROTTLE flag was created to handle exactly this >> circumstance, though with an NFS filesystem mounted from a >> local NFS server. It reduces the throttling on the lower >> layer so that it can proceed largely unthrottled. >> >> To demonstrate this, create a filesystem on a loop device >> and write (e.g. with dd) several large files which combine >> to consume significantly more than the limit set by >> /proc/sys/vm/dirty_ratio or dirty_bytes. Measure the total >> time taken. >> >> When I do this directly on a device (no loop device) the >> total time for several runs (mkfs, mount, write 200 files, >> umount) is fairly stable: 28-35 seconds. >> When I do this over a loop device the times are much worse >> and less stable. 52-460 seconds. Half below 100seconds, >> half above. >> When I apply this patch, the times become stable again, >> though not as fast as the no-loop-back case: 53-72 seconds. >> >> There may be room for further improvement as the total overhead still >> seems too high, but this is a big improvement. >> >> Signed-off-by: NeilBrown >> --- >> drivers/block/loop.c | 3 +++ >> 1 file changed, 3 insertions(+) >> >> diff --git a/drivers/block/loop.c b/drivers/block/loop.c >> index 0ecb6461ed81..a7e1dd215fc2 100644 >> --- a/drivers/block/loop.c >> +++ b/drivers/block/loop.c >> @@ -1694,8 +1694,11 @@ static void loop_queue_work(struct kthread_work *work) >> { >> struct loop_cmd *cmd = >> container_of(work, struct loop_cmd, work); >> + int oldflags = current->flags & PF_LESS_THROTTLE; >> >> + current->flags |= PF_LESS_THROTTLE; >> loop_handle_cmd(cmd); >> + current->flags = (current->flags & ~PF_LESS_THROTTLE) | oldflags; >> } > > You can do it against 'lo->worker_task' instead of doing it in each > loop_queue_work(), > and this flag needn't to be restored because the kernel thread is loop > specialized. > good point. I'll do that. Thanks, NeilBrown --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAljkcxgACgkQOeye3VZi gbkRCQ//ZXP+V7DavU/0u71sg3SPH2ddQBE1yJqZP+fs6YTWz0nsqA4sxoGsNe2T ldBJ2O2MFRlgFTmu2Fnk//KmLLyEMA4I9JjGElscfcK/cWHH3yVmC3WeVYwDZP/D uKn1EfmAI4nKdLs4mjSEDa+NZSV5A+RPwwDVETemsCpLFd8YNJ2fGDZcFBa9Obm2 kyuNfrsdVS/E7hPVJn/a37zZiY6ybIZc9972m1IzwQFZETDcY56Sad+9uHV09YHY n3XkYYVi4l7Mge3EvUUkiY620r+VmhCtwrT0demy9pt5airq9syMthKzlZzYle2g WZ2Q2xKgpmEDg4oZC1lFtirdgDPgOXYJfzC/3q1hwF8eB3607wzUMDmVBOTdXsTO vDQ8VaU5zHwKmR8BtSHTYcnFKKcVRnvffXsCqJ/EPjRppu+UQEaq3Jvpp73Unvnr Be0InhW7y/gVmquoLghocegLjKyWIDW0qhMcrYsA3S4LMAgys7RHcg1BaO8PmVsI xL7FZEmiaYiSAf+mSmcY/BUJOKui3AmTf8OX/HKj0Z5ShiNcsHjgUCwHPfW7Uadc hvns3Lnz7Ehu+pS/aMVxG/jqy+tFl2I/uNSVZzZr8ekrHr0auj5yRYE99yyhQe2x WkKFLRZdy2xUGb0noCWU5W72/7hHuMrmnppLNvo7Y7kqxtS4dmY= =BBID -----END PGP SIGNATURE----- --=-=-=--