On Oct 9, 2012, at 5:12 PM, Nikolay Denev <nde...@gmail.com> wrote:
>
> On Oct 4, 2012, at 12:36 AM, Rick Macklem <rmack...@uoguelph.ca> wrote:
>
>> Garrett Wollman wrote:
>>> <<On Wed, 3 Oct 2012 09:21:06 -0400 (EDT), Rick Macklem
>>> <rmack...@uoguelph.ca> said:
>>>
>>>>> Simple: just use a sepatate mutex for each list that a cache entry
>>>>> is on, rather than a global lock for everything. This would reduce
>>>>> the mutex contention, but I'm not sure how significantly since I
>>>>> don't have the means to measure it yet.
>>>>>
>>>> Well, since the cache trimming is removing entries from the lists, I
>>>> don't
>>>> see how that can be done with a global lock for list updates?
>>>
>>> Well, the global lock is what we have now, but the cache trimming
>>> process only looks at one list at a time, so not locking the list that
>>> isn't being iterated over probably wouldn't hurt, unless there's some
>>> mechanism (that I didn't see) for entries to move from one list to
>>> another. Note that I'm considering each hash bucket a separate
>>> "list". (One issue to worry about in that case would be cache-line
>>> contention in the array of hash buckets; perhaps NFSRVCACHE_HASHSIZE
>>> ought to be increased to reduce that.)
>>>
>> Yea, a separate mutex for each hash list might help. There is also the
>> LRU list that all entries end up on, that gets used by the trimming code.
>> (I think? I wrote this stuff about 8 years ago, so I haven't looked at
>> it in a while.)
>>
>> Also, increasing the hash table size is probably a good idea, especially
>> if you reduce how aggressively the cache is trimmed.
>>
>>>> Only doing it once/sec would result in a very large cache when
>>>> bursts of
>>>> traffic arrives.
>>>
>>> My servers have 96 GB of memory so that's not a big deal for me.
>>>
>> This code was originally "production tested" on a server with 1Gbyte,
>> so times have changed a bit;-)
>>
>>>> I'm not sure I see why doing it as a separate thread will improve
>>>> things.
>>>> There are N nfsd threads already (N can be bumped up to 256 if you
>>>> wish)
>>>> and having a bunch more "cache trimming threads" would just increase
>>>> contention, wouldn't it?
>>>
>>> Only one cache-trimming thread. The cache trim holds the (global)
>>> mutex for much longer than any individual nfsd service thread has any
>>> need to, and having N threads doing that in parallel is why it's so
>>> heavily contended. If there's only one thread doing the trim, then
>>> the nfsd service threads aren't spending time either contending on the
>>> mutex (it will be held less frequently and for shorter periods).
>>>
>> I think the little drc2.patch which will keep the nfsd threads from
>> acquiring the mutex and doing the trimming most of the time, might be
>> sufficient. I still don't see why a separate trimming thread will be
>> an advantage. I'd also be worried that the one cache trimming thread
>> won't get the job done soon enough.
>>
>> When I did production testing on a 1Gbyte server that saw a peak
>> load of about 100RPCs/sec, it was necessary to trim aggressively.
>> (Although I'd be tempted to say that a server with 1Gbyte is no
>> longer relevant, I recently recall someone trying to run FreeBSD
>> on a i486, although I doubt they wanted to run the nfsd on it.)
>>
>>>> The only negative effect I can think of w.r.t. having the nfsd
>>>> threads doing it would be a (I believe negligible) increase in RPC
>>>> response times (the time the nfsd thread spends trimming the cache).
>>>> As noted, I think this time would be negligible compared to disk I/O
>>>> and network transit times in the total RPC response time?
>>>
>>> With adaptive mutexes, many CPUs, lots of in-memory cache, and 10G
>>> network connectivity, spinning on a contended mutex takes a
>>> significant amount of CPU time. (For the current design of the NFS
>>> server, it may actually be a win to turn off adaptive mutexes -- I
>>> should give that a try once I'm able to do more testing.)
>>>
>> Have fun with it. Let me know when you have what you think is a good patch.
>>
>> rick
>>
>>> -GAWollman
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>
> My quest for IOPS over NFS continues :)
> So far I'm not able to achieve more than about 3000 8K read requests over NFS,
> while the server locally gives much more.
> And this is all from a file that is completely in ARC cache, no disk IO
> involved.
>
> I've snatched some sample DTrace script from the net : [
> http://utcc.utoronto.ca/~cks/space/blog/solaris/DTraceQuantizationNotes ]
>
> And modified it for our new NFS server :
>
> #!/usr/sbin/dtrace -qs
>
> fbt:kernel:nfsrvd_*:entry
> {
> self->ts = timestamp;
> @counts[probefunc] = count();
> }
>
> fbt:kernel:nfsrvd_*:return
> / self->ts > 0 /
> {
> this->delta = (timestamp-self->ts)/1000000;
> }
>
> fbt:kernel:nfsrvd_*:return
> / self->ts > 0 && this->delta > 100 /
> {
> @slow[probefunc, "ms"] = lquantize(this->delta, 100, 500, 50);
> }
>
> fbt:kernel:nfsrvd_*:return
> / self->ts > 0 /
> {
> @dist[probefunc, "ms"] = quantize(this->delta);
> self->ts = 0;
> }
>
> END
> {
> printf("\n");
> printa("function %-20s %@10d\n", @counts);
> printf("\n");
> printa("function %s(), time in %s:%@d\n", @dist);
> printf("\n");
> printa("function %s(), time in %s for >= 100 ms:%@d\n", @slow);
> }
>
> And here's a sample output from one or two minutes during the run of Oracle's
> ORION benchmark
> tool from a Linux machine, on a 32G file on NFS mount over 10G ethernet:
>
> [16:01]root@goliath:/home/ndenev# ./nfsrvd.d
> ^C
>
> function nfsrvd_access 4
> function nfsrvd_statfs 10
> function nfsrvd_getattr 14
> function nfsrvd_commit 76
> function nfsrvd_sentcache 110048
> function nfsrvd_write 110048
> function nfsrvd_read 283648
> function nfsrvd_dorpc 393800
> function nfsrvd_getcache 393800
> function nfsrvd_rephead 393800
> function nfsrvd_updatecache 393800
>
> function nfsrvd_access(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 4
> 1 | 0
>
> function nfsrvd_statfs(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 10
> 1 | 0
>
> function nfsrvd_getattr(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 14
> 1 | 0
>
> function nfsrvd_sentcache(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 110048
> 1 | 0
>
> function nfsrvd_rephead(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 393800
> 1 | 0
>
> function nfsrvd_updatecache(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 393800
> 1 | 0
>
> function nfsrvd_getcache(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 393798
> 1 | 1
> 2 | 0
> 4 | 1
> 8 | 0
>
> function nfsrvd_write(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 110039
> 1 | 5
> 2 | 4
> 4 | 0
>
> function nfsrvd_read(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 283622
> 1 | 19
> 2 | 3
> 4 | 2
> 8 | 0
> 16 | 1
> 32 | 0
> 64 | 0
> 128 | 0
> 256 | 1
> 512 | 0
>
> function nfsrvd_commit(), time in ms:
> value ------------- Distribution ------------- count
> -1 | 0
> 0 |@@@@@@@@@@@@@@@@@@@@@@@ 44
> 1 |@@@@@@@ 14
> 2 | 0
> 4 |@ 1
> 8 |@ 1
> 16 | 0
> 32 |@@@@@@@ 14
> 64 |@ 2
> 128 | 0
>
>
> function nfsrvd_commit(), time in ms for >= 100 ms:
> value ------------- Distribution ------------- count
> < 100 | 0
> 100 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1
> 150 | 0
>
> function nfsrvd_read(), time in ms for >= 100 ms:
> value ------------- Distribution ------------- count
> 250 | 0
> 300 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1
> 350 | 0
>
>
> Looks like the nfs server cache functions are quite fast, but extremely
> frequently called.
>
> I hope someone can find this information useful.
>
Here's another quick one :
#!/usr/sbin/dtrace -qs
#pragma D option quiet
fbt:kernel:nfsrvd_*:entry
{
self->trace = 1;
}
fbt:kernel:nfsrvd_*:return
/ self->trace /
{
@calls[probefunc] = count();
}
tick-1sec
{
printf("%40s | %s\n", "function", "calls per second");
printa("%40s %10@d\n", @calls);
clear(@calls);
printf("\n");
}
Showing the number of calls per second to the nfsrvd_* functions.
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