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nfsd: degraded slot-count more gracefully as allocation nears exhaustion.

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This original code in nfsd4_get_drc_mem() would hand out 30
slots (approximately NFSD_MAX_MEM_PER_SESSION bytes at slightly
over 2K per slot) to each requesting client until it ran out
of space, then it would possibly give one last client a reduced
allocation, then fail the allocation.

Since commit de766e5704 ("nfsd: give out fewer session slots as
limit approaches") the last 90 slots to be given to about 12
clients with quickly reducing slot counts (better than just 3
clients).  This still seems unnecessarily hasty.
A subsequent patch allows over-allocation so every client gets
at least one slot, but that might be a bit restrictive.

The requested number of nfsd threads is the best guide we have to the
expected number of clients, so use that - if it is at least 8.

256 threads on a 256Meg machine - which is a lot for a tiny machine -
would result in nfsd_drc_max_mem being 2Meg, so 8K (3 slots) would be
available for the first client, and over 200 clients would get more
than 1 slot.  So I don't think this change will be too debilitating on
poorly configured machines, though it does mean that a sensible
configuration is a little more important.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
This commit is contained in:
NeilBrown 2019-09-20 16:36:45 +10:00 committed by J. Bruce Fields
parent 7f49fd5d7a
commit 2030ca560c
1 changed files with 11 additions and 4 deletions
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15
fs/nfsd/nfs4state.c
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@ -1568,11 +1568,12 @@ static inline u32 slot_bytes(struct nfsd4_channel_attrs *ca)
* re-negotiate active sessions and reduce their slot usage to make
* room for new connections. For now we just fail the create session.
*/
static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca)
static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca, struct nfsd_net *nn)
{
u32 slotsize = slot_bytes(ca);
u32 num = ca->maxreqs;
unsigned long avail, total_avail;
unsigned int scale_factor;
spin_lock(&nfsd_drc_lock);
if (nfsd_drc_max_mem > nfsd_drc_mem_used)
@ -1586,12 +1587,18 @@ static u32 nfsd4_get_drc_mem(struct nfsd4_channel_attrs *ca)
total_avail = 0;
avail = min((unsigned long)NFSD_MAX_MEM_PER_SESSION, total_avail);
/*
* Never use more than a third of the remaining memory,
* Never use more than a fraction of the remaining memory,
* unless it's the only way to give this client a slot.
* The chosen fraction is either 1/8 or 1/number of threads,
* whichever is smaller. This ensures there are adequate
* slots to support multiple clients per thread.
* Give the client one slot even if that would require
* over-allocation--it is better than failure.
*/
avail = clamp_t(unsigned long, avail, slotsize, total_avail/3);
scale_factor = max_t(unsigned int, 8, nn->nfsd_serv->sv_nrthreads);
avail = clamp_t(unsigned long, avail, slotsize,
total_avail/scale_factor);
num = min_t(int, num, avail / slotsize);
num = max_t(int, num, 1);
nfsd_drc_mem_used += num * slotsize;
@ -3188,7 +3195,7 @@ static __be32 check_forechannel_attrs(struct nfsd4_channel_attrs *ca, struct nfs
* Note that we always allow at least one slot, because our
* accounting is soft and provides no guarantees either way.
*/
ca->maxreqs = nfsd4_get_drc_mem(ca);
ca->maxreqs = nfsd4_get_drc_mem(ca, nn);
return nfs_ok;
}