Linux-NFS Archive on lore.kernel.org
 help / color / Atom feed
From: Alkis Georgopoulos <alkisg@gmail.com>
To: Trond Myklebust <trondmy@gmail.com>,
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Subject: Re: rsize,wsize=1M causes severe lags in 10/100 Mbps
Date: Fri, 20 Sep 2019 12:25:16 +0300
Message-ID: <0213704b-3930-5be6-bd3d-dbaabc24a270@gmail.com> (raw)
In-Reply-To: <20190919221601.GA30751@cosmos.ssec.wisc.edu>

On 9/20/19 1:16 AM, Daniel Forrest wrote:
>>> What may be happening here is something I have noticed with glibc.
>>>
>>> - statfs reports the rsize/wsize as the block size of the filesystem.
>>>
>>> - glibc uses the block size as the default buffer size for
>>> fread/fwrite.
>>>
>>> If an application is using fread/fwrite on an NFS mounted file with
>>> an rsize/wsize of 1M it will try to fill a 1MB buffer.
>>>
>>> I have often changed mounts to use rsize/wsize=64K to alleviate this.
>>
>> That sounds like an abuse of the filesystem block size. There is
>> nothing in the POSIX definition of either fread() or fwrite() that
>> requires glibc to do this:
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/fread.html
>> https://pubs.opengroup.org/onlinepubs/9699919799/functions/fwrite.html
>>
> 
> It looks like this was fixed in glibc 2.25:
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=4099


This is likely not the exact issue I'm experiencing, as I'm testing e.g.
with glibc 2.27-3ubuntu1 on Ubuntu 18.04 and kernel 5.0.

New benchmark, measuring the boot time of a netbooted client,
from right after the kernel is loaded to the display manager screen:

1) On 10 Mbps:
a) tcp,timeo=600,rsize=32K: 304 secs
b) tcp,timeo=600,rsize=1M: 618 secs

2) On 100 Mbps:
a) tcp,timeo=600,rsize=32K: 40 secs
b) tcp,timeo=600,rsize=1M: 84 secs

3) On 1000 Mbps:
a) tcp,timeo=600,rsize=32K: 20 secs
b) tcp,timeo=600,rsize=1M: 24 secs

32K is always faster, even on full gigabit.
Disk access on gigabit was *significantly* faster to result in 4 seconds 
lower boot time. In the 10/100 cases, rsize=1M is pretty much unusable.
There are no writes involved, they go in a local tmpfs/overlayfs.
Would it make sense for me to measure the *boot bandwidth* in each case, 
to see if more things (readahead) are downloaded with rsize=1M?

I can do whatever benchmarks and test whatever parameters you tell me 
to, but I do not know the NFS/kernel internals to be able to explain why 
this happens.

The reason I investigated this is because I developed the new version of 
ltsp.org (GPL netbooting software), where we switched from 
squashfs-over-NBD to squashfs-over-NFS, and netbooting was extremely 
slow until I lowered rsize to 32K, so I thought I'd share my findings in 
case it makes a better default for everyone (or reveals a problem 
elsewhere).
With rsize=32K, squashfs-over-NFS is as speedy as squashfs-over-NBD, but 
a lot more stable.

Of course the same rsize findings apply for NFS /home too (without 
nfsmount), or for just transferring large or small files, not just for /.

Btw, 
https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt 
says the kernel nfsroot defaults are timeo=7,rsize=4096,wsize=4096. This 
is about the internal kernel netbooting support, not using klibc 
nfsmount; but I haven't tested it as it would involve compiling a kernel 
with my NIC driver.

Thank you,
Alkis Georgopoulos
LTSP developer

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-19  7:29 rsize,wsize=1M causes severe lags in 10/100 Mbps, what sets those defaults? Alkis Georgopoulos
2019-09-19 15:08 ` Trond Myklebust
2019-09-19 15:58   ` rsize,wsize=1M causes severe lags in 10/100 Mbps Alkis Georgopoulos
2019-09-19 16:11     ` Trond Myklebust
2019-09-19 19:21       ` Alkis Georgopoulos
2019-09-19 19:51         ` Trond Myklebust
2019-09-19 19:57           ` Alkis Georgopoulos
2019-09-19 20:05             ` Trond Myklebust
2019-09-19 20:20               ` Alkis Georgopoulos
2019-09-19 20:40                 ` Trond Myklebust
2019-09-19 21:19                   ` Daniel Forrest
2019-09-19 21:42                     ` Trond Myklebust
2019-09-19 22:16                       ` Daniel Forrest
2019-09-20  9:25                         ` Alkis Georgopoulos [this message]
2019-09-20  9:48                           ` Alkis Georgopoulos
2019-09-20 10:04                             ` Alkis Georgopoulos
2019-09-21  7:52                               ` Alkis Georgopoulos
2019-09-21  7:59                                 ` Alkis Georgopoulos
2019-09-21 11:02                                   ` Alkis Georgopoulos

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=0213704b-3930-5be6-bd3d-dbaabc24a270@gmail.com \
    --to=alkisg@gmail.com \
    --cc=linux-nfs@vger.kernel.org \
    --cc=trondmy@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-NFS Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-nfs/0 linux-nfs/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-nfs linux-nfs/ https://lore.kernel.org/linux-nfs \
		linux-nfs@vger.kernel.org
	public-inbox-index linux-nfs

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-nfs


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git