Date: Sun, 15 Apr 2018 13:10:24 +0200 From: Niels Kobschaetzki <niels@kobschaetzki.net> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: "freebsd-net@freebsd.org" <freebsd-net@freebsd.org> Subject: Re: High rate of NFS cache misses after upgrading from 10.3-prerelease to 11.1-release Message-ID: <36907CE0-EAD3-4E11-8023-5BCEA1239813@kobschaetzki.net> In-Reply-To: <YQBPR0101MB1042087832CE6FDCCA3B4216DDB20@YQBPR0101MB1042.CANPRD01.PROD.OUTLOOK.COM> References: <ce3712c0-626e-c8f2-3bba-933cf359bcef@kobschaetzki.net> <YQBPR0101MB1042D2F0CE2575EB4F17588ADDB20@YQBPR0101MB1042.CANPRD01.PROD.OUTLOOK.COM> <f3cea179-75d7-916b-68d1-61fe75c0bb80@kobschaetzki.net> <YQBPR0101MB1042087832CE6FDCCA3B4216DDB20@YQBPR0101MB1042.CANPRD01.PROD.OUTLOOK.COM>
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> On 15. Apr 2018, at 01:18, Rick Macklem <rmacklem@uoguelph.ca> wrote:
>
> Niels Kobschätzki wrote:
>>> On 04/14/2018 03:49 AM, Rick Macklem wrote:
>>> Niels Kobschätzki wrote:
>>>> sorry for the cross-posting but so far I had no real luck on the forum
>>>> or on question, thus I want to try my luck here as well.
>>> I read email lists but don't do the other stuff, so I just saw this yesterday.
>>> Short answer, I haven't a clue why cache hits rate would have changed.
>>>
>>> The code that decides if there is a hit/miss for the attribute cache is in
>>> ncl_getattrcache() and the code hasn't changed between 10.3->11.1,
>>> except the old code did a mtx_lock(&Giant), but I can't imagine how that
>>> would affect the code.
>>>
>>> You might want to:
>>> # sysctl -a | fgrep vfs.nfs
>>> for both the 10.3 and 11.1 systems, to check if any defaults have somehow
>>> been changed. (I don't recall any being changed, but??)
>>
>> I did that and there did nothing change.
>>
>>> If you go into ncl_getattrcache() {it's in sys/fs/nfsclient/nfs_clsubs.c}
>>> and add a printf() for "time_second" and "np->n_mtime.tv_sec" near the
>>> top, where it calculates "timeo" from it.
>>> Running this hacked kernel might show you if either of these fields is bogus.
>>> (You could then printf() "timeo" and "np->n_attrtimeo" just before the "if"
>>> clause that increments "attrcache_misses", which is where the cache misses
>>> happen to see why it is missing the cache.)
>>> If you could do this for the 10.3 kernel as well, this might indicate why the
>>> miss rate has increased?
>>
>> I will do this next week. On monday we switch for other reasons to other
>> nfs-servers and when we see that they run stable, I will do this next.
> With a miss rate of 2.7%, I doubt printing the above will help. I thought
> you were seeing a high miss rate.
It is low but increased by nearly a factor of 1000 to before. I hope the print will help. Just a lot of grepping through wherever I can get this data.
>> Btw. I calculated now the percentages. The old servers had a attr miss
>> rate of something like 0.004%, while the upgraded one has more like
>> 2.7%. This is till low from what I've read (I remember that you should
>> start adjusting acreg* when you hit more than 40% misses) but far higher
>> than before.
> You could try increasing acregmin, acregmax and see if the misses are reduced.
> (The only risk with increasing the cache timeout is that, if another client changes
> the attributes, then the client will use stale ones for longer. Usually, this doesn't
> cause serious problems.)
I tried that and it had exactly no effect
> To be honest, a Getattr RPC is pretty low overhead, so I doubt the increase
> to 2.7% will affect your application's performance, but it is interesting that
> it increased.
It is a website with quite some traffic handles by three webservers behind a pair of loadbalancers.
We see a loss of 20% in speed(TTFB reduced by 100ms; sounds not a lot but Google et al doesn’t like it at all) after upgrading to 11.1 with a combined upgrade to php7.1. On another server without NFS that upgrade improved performance considerably (I was told ca 30% by the front end-dev)
> You might also try increasing acdirmin, acdirmax in case it is the directory
> attributes that are having cache misses.
I did that, too
> Oh, and check that your time of day clocks are in sync with the server,
> since the caches are time based, since there is no cache coherency protocol
> in NFS.
I checked that. All three frontends are using the same server for ntp
Thanks so far,
Niels
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