On 9/19/24 16:57, Thomas Lamprecht wrote:
Am 19/09/2024 um 14:45 schrieb Dominik Csapak:
On 9/19/24 14:01, Thomas Lamprecht wrote:
Am 19/09/2024 um 11:52 schrieb Dominik Csapak:
by default libfuse2 limits writes to 4k size, which means that on writes
bigger than that, we do a whole write cycle for each 4k block that comes
in. To avoid that, add the option 'big_writes' to allow writes bigger
than 4k at once.
This should improve pmxcfs performance for situations where we often
write large files (e.g. big ha status) and maybe reduce writes to disk.
Should? Something like before/after for benchmark numbers, flamegraphs
would be really good to have, without those it's rather hard to discuss
this, and I'd like to avoid having to do those, or check the inner workings
of the affected fuse userspace/kernel code paths here myself.
well I mean the code change is relatively small and the result is rather clear:
Well sure the code change is just setting an option... But the actual change is
abstracted away and would benefit from actually looking into..
in the current case we have the following calls from pmxcfs (shortened for
e-mail)
when writing a single 128k block:
(dd if=... of=/etc/pve/test bs=128k count=1)
Better than nothing but still no actual numbers (reduced time, reduced write amp
in combination with sqlite, ...), some basic analysis over file/write size
distribution
on a single node and (e.g. three node) cluster, ...
If that's all obvious for you then great, but as already mentioned in the past,
I
want actual data in commit messages for such stuff, and I cannot really see a
downside
of having such numbers.
Again, as is I'm not really seeing what's to discuss, you send it as RFC after
all.
[...]
so a factor of 32 less calls to cfs_fuse_write (including memdb_pwrite)
That can be huge or not so big at all, i.e. as mentioned above, it would we
good to
measure the impact through some other metrics.
And FWIW, I used bpftrace to count [0] with an unpatched pmxcfs, there I get
the 32 calls to cfs_fuse_write only for a new file, overwriting the existing
file again with the same amount of data (128k) just causes a single call.
I tried using more data (e.g. from 128k initially to 256k or 512k) and it's
always the data divided by 128k (even if the first file has a different size)
We do not override existing files often, but rather write to a new file and
then rename, but still quite interesting and IMO really showing that just
because this is 1 +-1 line change it doesn't necessarily have to be trivial
and obvious in its effects.
[0]: bpftrace -e 'u:cfs_fuse_write /str(args->path) == "/test"/ {@ = count();} END {
print(@) }' -p "$(pidof pmxcfs)"
If we'd change to libfuse3, this would be a non-issue, since that option
got removed and is the default there.
I'd prefer that. At least if done with the future PVE 9.0, as I do not think
it's a good idea in the middle of a stable release cycle.
why not this change now, and the rewrite to libfuse3 later? that way we can
have some improvements now too...
Because I want some actual data and reasoning first, even if it's quite likely
that this improves things Somehow™, I'd like to actually know in what metrics
and by how much (even if just an upper bound due to the benchmark or some
measurement being rather artificial).
I mean you name the big HA status, why not measure that for real? like, probably
among other things, in terms of bytes hitting the block layer, i.e. the actual
backing disk from those requests as then we'd know for real if this can reduce
the write load there, not just that it maybe should.
hi,
first i just wanted to say I'm sorry for my snarky comment about not needing to
test
performance for such code. You're right, any insight we can gain there
is good and we (I!) should take the time to do that, even if the
change looks "obvious" like it does here
so i did some benchmarks (mostly disk writes) and wrote the short script below
(maybe we can reuse that?)
----8<----
use strict;
use warnings;
use PVE::Tools;
my $size = shift;
sub get_bytes_written {
my $fh = IO::File->new("/proc/diskstats", "r");
die if !$fh;
my $bytes = undef;
while (defined(my $line = <$fh>)) {
if ($line =~ m/sdb/) {
my @fields = split(/\s+/, $line);
$bytes = $fields[10] * 512;
}
}
return $bytes;
}
sub test_write {
my ($k) = @_;
system("rm /etc/pve/testfile");
my $data = "a"x($k*1024);
system("sync; echo -n 3> /proc/sys/vm/drop_caches");
my $bytes_before = get_bytes_written();
PVE::Tools::file_set_contents("/etc/pve/testfile", $data);
system("sync; echo -n 3> /proc/sys/vm/drop_caches");
my $bytes_after = get_bytes_written();
return $bytes_after - $bytes_before;
}
$size //= 128;
my $written = test_write($size) / 1024;
print("$written\n");
---->8----
to simulate our real write patterns with varying file sizes
i installed a fresh pve, turned off pvestatd and put /var/lib/pve-cluster on
it's own disk
(/dev/sdb), so diskstats only contains writes from the pmxcfs
the results are below (all sizes are kbytes, ran them multiple times, but they
seem to be consistent)
data size written (old) amplification (old) written (new) amplification
(new)
1 56 56 56 56
2 72 36 76 38
4 84 21 88 22
8 144 18 104 13
16 236 14 160 10
32 532 16 324 10
64 1496 23 836 13
128 6616 51 3848 30
256 20600 80 10568 41
512 87296 170 43416 84
1024 388460 379 197032 192
for smaller writes there seems to be a minimum overhead of ~50-100 kbytes
for big files with have a massive amplification of > 100x
my patch does seem to make a difference for files >4k
but the biggest surprise here is that the write amplification
is not linear, but increases with an increase in bytes we want to write
so e.g. going from 128k -> 256k file write we don't just write double the
amount,
but 3x to 4x as much.
i also produced a flamegraph according to
https://www.brendangregg.com/FlameGraphs/cpuflamegraphs.html, but that showed virtually no change
between versions without
and with my patch (if one has a good svg hoster, i can post them ofc)
so, tl;dr
for small writes does not make that much of a difference, but
we can save ~half the writes for large files in the pmxcfs
(which e.g. a ha-state file could do)
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