Austin S. Hemmelgarn posted on Mon, 18 Jan 2016 07:48:13 -0500 as
excerpted:

On 2016-01-17 22:51, Duncan wrote:

quoted

Checking a bit more my understanding, since you brought up the btrfs
"commit=" mount option.

I knew about the option previously, and obviously knew it worked in the
same context as the page-cache stuff, but in my understanding the btrfs
"commit=" mount option operates at the filesystem layer, not the
general filesystem-vm layer controlled by /proc/sys/vm/dirty_*.  In my
understanding, therefore, the two timeouts could effectively be added,
yielding a maximum 1 minute (30 seconds btrfs default commit time plus
30 seconds vm expiry) commit time.

In a way, yes, except the commit option controls when a transaction is
committed, and thus how often the log tree gets cleared.  It's
essentially saying 'ensure the filesystem is consistent without
replaying a log at least this often'.  AFAIUI, this doesn't guarantee
that you'll go that long without a transaction, but puts an upper bound
on it.  Looking at it another way, it pretty much says that you don't
care about losing the last n seconds of changes to the FS.

Thanks.  That's the way I was treating it.

The sysctl values are a bit different, and control how long the kernel
will wait in the VFS layer to try and submit a larger batch of writes at
once, so that the block layer has more it can try to merge, and
hopefully things get written out faster as a result.  IOW, it's a knob
to control the VFS level write-back caching to try and tune for
performance.  This also ties in with
/proc/sys/vm/dirty_writeback_centisecs, which is how often after the
expiration hits that the kernel will flush a chunk of the cache, and
/proc/sys/vm/dirty_{background,}_{bytes,ratio} which puts an upper limit
on how much data will be buffered before trying to flush it out to
persistent storage.  You almost certainly want to change these, as they
defaults to 10% of system RAM, which is why it often takes a ridiculous
amount of time to unmount a flash drive that's been written to a lot.
dirty_{ratio,bytes} control the per-process limit, and
dirty_background_{ratio,bytes} control the system-wide limit.

Got that too, and yes, I've been known to recommend to others changes to 
the now-days ridiculous 10% of system RAM buffer thing, as well. =:^)  
Random writes to spinning rust in particular may be 30 MiB/sec real-
world, and 10% of 16 GiB is 1.6 GiB, 50-some seconds worth of writeback.  
When the timeout is 30 seconds and the backlog is nearly double that, 
something's wrong.  I set mine to 3% foreground (~ half a gig @ 16 GiB) 
and 1% (~160 MiB) background when I upgraded to 16 GiB RAM, tho now I 
have fast SSDs, but didn't see a need to boost it back up, as half a GiB 
is quite enough to have unsynced in case of a crash anyway.

(Obviously once RAM goes above ~16 GiB, for systems not yet on fast SSD, 
the bytes values begin to make more sense to use than ratio, as 1% of RAM 
is simply no longer fine enough granularity.  But 1% of 16 GiB is ~163 
MiB, ~5 seconds worth @ 30 MiB/sec, so fine /enough/... barely.  The 3% 
foreground figure is then ~16 seconds worth of writeback, a bit 
uncomfortable if you're waiting on it, but comfortably below the 30 
second timeout and still at least tolerable in human terms, so not /too/ 
bad.  And as I said, for me the system and /home are now on fast SSD, so 
in practice the only time I'm worrying about spinning rust transfer 
backlogs is on the media and backups drive, which is still spinning 
rust.  And it's tolerable there, so the ratio knobs continue to be fine, 
for my own use.)

quoted

But that has always been an unverified on my part fuzzy assumption. 
The two times could be the same layer, with the btrfs mount option
being a per-filesystem method of controlling the same thing that
/proc/sys/vm/ dirty_expire_centisecs controls globally (as you seemed
to imply above), or the two could be different layers but with the
countdown times overlapping, both of which would result in a 30-second
total timeout, instead of the 30+30=60 that I had assumed.

The two timers do overlap.

Good to have it verified. =:^)  The difference between 30 seconds and a 
minute's worth of work lost in a crash can be quite a lot, if one was 
copying a big set of small files at the time.

quoted

And while we're at it, how does /proc/sys/vm/vfs_cache_pressure play
into all this?  I know the dirty_* and how the dirty_*bytes vs.
dirty_*ratio vs. dirty_*centisecs thing works, but don't quite
understand how vfs_cache_pressure fits in with dirty_*.

vfs_cache_pressure controls how likely the kernel is to drop clean pages
(the documentation says just dentries and inodes, but I'm relatively
certain it's anything in the VFS cache) from the VFS cache to get memory
to allocate.  The higher this is, the more likely the VFS cache is to
get invalidated.  In general, you probably want to increase this on
systems that have fast storage (like SSD's or really good SAS RAID
arrays, 150 is usually a decent start), and decrease it if you have
really slow storage (Like a Raspberry Pi for example).  Setting this too
low (below about 50) however, will give you a very high chance of
getting an OOM condition.

So vfs_cache_pressure only applies if you're out of "free" memory, and 
the kernel has to decide whether to dump cache or OOM, correct?  On 
systems with enough memory, and with stuff like the local package cache 
and/or multimedia on separate partitions that are mounted only when 
needed and unmounted when not, so actual system-and-apps plus buffers 
plus cache memory generally stays reasonably below total RAM, with 
reasonable ulimits and tmpfs maximum sizes set so apps can't go hog-wild, 
there's zero cache pressure so this setting doesn't apply at all... 
unless/until there's a bad kernel leak and/or several apps go somewhat 
wild, plus something's maximizing a few of those tmpfs, all at once, of 
course.

(As I write this system/app memory usage is ~2350 MiB, buffers 4 MiB, 
cache 7321 MiB, total usage ~9680 MiB, on a 16 GiB system.  That's with 
about three days uptime, after mounting the packages partition and 
remounting / rw and doing a bunch of builds, then umounting the pkgs 
partition, killing X and running a lib_users check to ensure no services 
are running on outdated deleted libs and need restarted, remounting / ro, 
and restarting X.  At some point I had the media partition mounted too, 
but now it's unmounted again, dropping that cache.  So in addition to 
cache memory which /could/ be dumped if I had to, I have 6+ GiB of 
entirely idle unused memory.  Nice as I don't have swap configured, so if 
I'm out of RAM, I'm out, but there's a lot of cache to dump first before 
it gets that bad.  Meanwhile, zero cache pressure, and 6+ GiB of spare 
RAM to use for apps/tmpfs/cache if I need it, before any cache dumps at 
all! =:^)

Documentation/sysctl/vm.txt in the kernel sources covers them, although
the documentation is a bit sparse even there.

I knew the kernel's proc documentation in Documentation/filesystems/
proc.txt, plus whatever outside resource it was that originally got me 
looking into the whole thing in the first place, I had the /proc/sys/vm/
dirty_* files and their usage covered.  But the sysctl/* doc files and 
the the vfs_cache_pressure proc file, not so much, and as I said I didn't 
understand how the btrfs commit= mount option fit into all of this.  So 
now I have a rather better understanding of how it all fits together. =:^)

Thanks.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman