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SubjectRe: [patch] nfsd optimizations for test10 (recoded to use list_head)
On Sunday November 12, ying@almaden.ibm.com wrote:
>
> Hi,
>
> This is the recoded racache that uses list_head for several lists, e.g.,
> lru and free lists. I have tested it under SPEC SFS runs, and several other
> NFS loads myself.

Ok, I have taken a closer look at this code:

1/ Why did you change nfsd_busy into an atomic_t? It is only ever
used or updated inside the Big-Kernel-Lock, so it doesn't need
to be atomic.

2/ Your new nfsd_racache_init always allocates a new cache, were as
the current one checks first to see if it has already been
allocated.
This is important because it is quite legal to run "rpc.nfsd"
multiple times. Subsequent invocations serve to change the number
of nfsd threads running.

3/ You currently allocate a single slab of memory for all of the
"struct raparms". Admittedly this is what the old code did, but I
don't think that it is really necessary, and calling kmalloc
multiple times would work just a well and would (arguably) be
clearer.

4/ small point: you added a hash table as the comment suggests might
be needed, but you didn't change the comment accordingly:-)

5/ the calls to spin_lock/spin_unlock in nfsd_racache_init seem
pointless. At this point, nothing else could possibly be accessing
the racache, and if it was you would have even bigger problems.
ditto for nfsd_racache_shutdown

6/ The lru list is now a list.h list, but the hash lists aren't. Why
is that?

7/ The old code kept a 'use' count for each cache entry to make sure
that an entry was not reused while it was in use. You have dropped
this. Now because of the lru ordering, and because each thread can
use at most one entry, you wont have a problem if there are more
cache entries than threads, and you currently have 2048 entries
configured which is greater than NFSD_MAXSERVS. However I think it
would be best if this dependancy were made explicit.
Maybe the call to nfsd_racache_init should tell the racache how
many threads are being started, and nfsd_racache_init should record
how many cache entries have been alloced, and it could alloc some
more if needed.

8/ I would like the stats collected to tell me a bit more about what
was going on. If find simple hit/miss numbers nearly useless, as
you expect many lookups to be misses anyway (first time a file is
accessed) but you don't know what percentage.
As a first approximation, I would like to only count a miss if the
seek address was > 0.
What would be really nice would be to get stats on how long entries
stayed in the cache between last use and re-use. If we stored a
'last-use' time in each entry, and on reuse, kept count of which
range the age was is:

0-62 msec
63-125 msec
125-250 msec
250-500 msec
500-1000 msec
1-2 sec
2-4 sec
4-8 sec
8-16 sec
16-32 sec

This obviously isn't critical, but it would be nice to be able
to see how the cache was working.


9/ Actually, you don't need the spinlock at all, and nfsd is currently
all under the BigKernelLock, but it doesn't hurt to have it around
the nfsd_get_raparms function because we hopefully will get rid of
the BKL one day.

NeilBrown





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