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SubjectRe: [PATCH 00/17] [RFC] soft and dynamic dirty throttling limits
On Wed, Oct 13, 2010 at 11:07:33AM +0800, Dave Chinner wrote:
> On Tue, Oct 12, 2010 at 10:17:16AM -0400, Christoph Hellwig wrote:
> > Wu, what's the state of this series? It looks like we'll need it
> > rather sooner than later - try to get at least the preparations in
> > ASAP would be really helpful.
>
> Not ready in it's current form. This load (creating millions of 1
> byte files in parallel):
>
> $ /usr/bin/time ./fs_mark -D 10000 -S0 -n 100000 -s 1 -L 63 \
> > -d /mnt/scratch/0 -d /mnt/scratch/1 \
> > -d /mnt/scratch/2 -d /mnt/scratch/3 \
> > -d /mnt/scratch/4 -d /mnt/scratch/5 \
> > -d /mnt/scratch/6 -d /mnt/scratch/7
>
> Locks up all the fs_mark processes spinning in traces like the
> following and no further progress is made when the inode cache
> fills memory.

I reproduced the problem on a 6G/8p 2-socket 11-disk box.

The root cause is, pageout() is somehow called with low scan priority,
which deserves more investigation.

The direct cause is, balance_dirty_pages() then keeps nr_dirty too low,
which can be improved easily by not pushing down the soft dirty limit
to less than 1-second worth of dirty pages.

My test box has two nodes, and their memory usage are rather unbalanced:
(Dave, maybe you have NUMA setup too?)

root@wfg-ne02 ~# cat /sys/devices/system/node/node0/meminfo
root@wfg-ne02 ~# cat /sys/devices/system/node/node1/meminfo

Node 0 Node 1
------------------------------------------
MemTotal: 3133760 kB 3145728 kB
==> MemFree: 453016 kB 2283884 kB
==> MemUsed: 2680744 kB 861844 kB
Active: 436436 kB 9744 kB
Inactive: 846400 kB 37196 kB
Active(anon): 113304 kB 1588 kB
Inactive(anon): 412 kB 0 kB
Active(file): 323132 kB 8156 kB
Inactive(file): 845988 kB 37196 kB
Unevictable: 0 kB 0 kB
Mlocked: 0 kB 0 kB
Dirty: 244 kB 0 kB
Writeback: 0 kB 0 kB
FilePages: 1169832 kB 45352 kB
Mapped: 9088 kB 0 kB
AnonPages: 113596 kB 1588 kB
Shmem: 416 kB 0 kB
KernelStack: 1472 kB 8 kB
PageTables: 2600 kB 0 kB
NFS_Unstable: 0 kB 0 kB
Bounce: 0 kB 0 kB
WritebackTmp: 0 kB 0 kB
Slab: 1133616 kB 701972 kB
SReclaimable: 902552 kB 693048 kB
SUnreclaim: 231064 kB 8924 kB
HugePages_Total: 0 0
HugePages_Free: 0 0
HugePages_Surp: 0 0
And somehow pageout() is called with very low scan priority, hence
the vm_dirty_pressure introduced in patch "mm: lower soft dirty limits on
memory pressure" goes all the way down to 0, which makes balance_dirty_pages()
start aggressive dirty throttling.

root@wfg-ne02 ~# cat /debug/vm/dirty_pressure
0
root@wfg-ne02 ~# echo 1024 > /debug/vm/dirty_pressure
After restoring vm_dirty_pressure the performance immediately restores:

# vmmon nr_free_pages nr_anon_pages nr_file_pages nr_dirty nr_writeback nr_slab_reclaimable slabs_scanned

nr_free_pages nr_anon_pages nr_file_pages nr_dirty nr_writeback nr_slab_reclaimable slabs_scanned
870915 13165 337210 1602 8394 221271 2910208
869924 13206 338116 1532 8293 221414 2910208
868889 13245 338977 1403 7764 221515 2910208
867892 13359 339669 1327 8071 221579 2910208
--- vm_dirty_pressure restores from here on ---------------------------------------------------------------------------
866354 13358 341162 2290 8290 221665 2910208
863627 13419 343259 4014 8332 221833 2910208
861008 13662 344968 5854 8333 222092 2910208
858513 13601 347019 7622 8333 222371 2910208
855272 13693 348987 9449 8333 223301 2910208
851959 13789 350898 11287 8333 224273 2910208
848641 13878 352812 13060 8333 225223 2910208
845398 13967 354822 14854 8333 226193 2910208
842216 14062 356749 16684 8333 227148 2910208
838844 14152 358862 18500 8333 228129 2910208
835447 14245 360678 20313 8333 229084 2910208
832265 14338 362561 22117 8333 230058 2910208
829098 14429 364710 23906 8333 231005 2910208
825609 14520 366530 25726 8333 231971 2910208
# dstat
----total-cpu-usage---- -dsk/total- -net/total- ---paging-- ---system--
usr sys idl wai hiq siq| read writ| recv send| in out | int csw
0 6 82 0 0 12| 0 2240k| 766B 8066B| 0 0 |1435 1649
0 4 85 0 0 11| 0 2266k| 262B 436B| 0 0 |1141 1055
0 5 83 0 0 12| 0 2196k| 630B 7132B| 0 0 |1144 1053
0 6 81 0 0 13| 0 2424k|1134B 20k| 0 0 |1284 1282
0 7 81 0 0 12| 0 2152k| 628B 4660B| 0 0 |1634 1944
0 4 84 0 0 12| 0 2184k| 192B 580B| 0 0 |1133 1037
0 4 84 0 0 12| 0 2440k| 192B 564B| 0 0 |1197 1124
--- vm_dirty_pressure restores from here on -----------------------------------
0 51 35 0 0 14| 112k 6718k| 20k 17k| 0 0 |2539 1478
1 83 0 0 0 17| 0 13M| 252B 564B| 0 0 |3221 1270
0 78 6 0 0 16| 0 15M|1434B 12k| 0 0 |3596 1590
0 83 1 0 0 16| 0 13M| 324B 4154B| 0 0 |3318 1374
0 80 4 1 0 16| 0 14M|1706B 9824B| 0 0 |3469 1632
0 76 5 1 0 18| 0 15M| 636B 4558B| 0 0 |3777 1940
0 71 9 1 0 19| 0 17M| 510B 3068B| 0 0 |4018 2277
Thanks,
Fengguang


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