Hello!
Writing back single file pages during reclaim exhibits bad IO
patterns, but we can't just stop doing that before the VM has other
means to ensure the pages in a zone are reclaimable.
Over time there were several suggestions of at least doing
write-around of the pages in inode-proximity when the need arises to
clean pages during memory pressure. But even that would interrupt
writeback from the flushers, without any guarantees that the nearby
inode-pages are even sitting on the same troubled zone.
The reason why dirty pages reach the end of LRU lists in the first
place is in part because the dirty limits are a global restriction
while most systems have more than one LRU list that are different in
size. Multiple nodes have multiple zones have multiple file lists but
at the same time there is nothing to balance the dirty pages between
the lists except for reclaim writing them out upon encounter.
With around 4G of RAM, a x86_64 machine of mine has a DMA32 zone of a
bit over 3G, a Normal zone of 500M, and a DMA zone of 15M.
A linear writer can quickly fill up the Normal zone, then the DMA32
zone, throttled by the dirty limit initially. The flushers catch up,
the zones are now mostly full of clean pages and memory reclaim kicks
in on subsequent allocations. The pages it frees from the Normal zone
are quickly filled with dirty pages (unthrottled, as the much bigger
DMA32 zone allows for a huge number of dirty pages in comparison to
the Normal zone). As there are also anon and active file pages on the
Normal zone, it is not unlikely that a significant amount of its
inactive file pages are now dirty [ foo=zone(global) ]:
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=112313(821289) active=9942(10039) isolated=27(27) dirty=59709(146944) writeback=739(4017)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=111102(806876) active=9925(10022) isolated=32(32) dirty=72125(146914) writeback=957(3972)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=110493(803374) active=9871(9978) isolated=32(32) dirty=57274(146618) writeback=4088(4088)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=111957(806559) active=9871(9978) isolated=32(32) dirty=65125(147329) writeback=456(3866)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=110601(803978) active=9860(9973) isolated=27(27) dirty=63792(146590) writeback=61(4276)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=111786(804032) active=9860(9973) isolated=0(64) dirty=64310(146998) writeback=1282(3847)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=111643(805651) active=9860(9982) isolated=32(32) dirty=63778(147217) writeback=1127(4156)
reclaim: blkdev_writepage+0x0/0x20 zone=Normal inactive=111678(804709) active=9859(10112) isolated=27(27) dirty=81673(148224) writeback=29(4233)
[ These prints occur only once per reclaim invocation, so the actual
->writepage calls are more frequent than the timestamp may suggest. ]
In the scenario without the Normal zone, first the DMA32 zone fills
up, then the DMA zone. When reclaim kicks in, it is presented with a
DMA zone whose inactive pages are all dirty -- and dirtied most
recently at that, so the flushers really had abysmal chances at making
some headway:
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=776(430813) active=2(2931) isolated=32(32) dirty=814(68649) writeback=0(18765)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=726(430344) active=2(2931) isolated=32(32) dirty=764(67790) writeback=0(17146)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=729(430838) active=2(2931) isolated=32(32) dirty=293(65303) writeback=468(20122)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=757(431181) active=2(2931) isolated=32(32) dirty=63(68851) writeback=731(15926)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=758(432808) active=2(2931) isolated=32(32) dirty=645(64106) writeback=0(19666)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=726(431018) active=2(2931) isolated=32(32) dirty=740(65770) writeback=10(17907)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=697(430467) active=2(2931) isolated=32(32) dirty=743(63757) writeback=0(18826)
reclaim: xfs_vm_writepage+0x0/0x4f0 zone=DMA inactive=693(430951) active=2(2931) isolated=32(32) dirty=626(54529) writeback=91(16198)
The idea behind this patch set is to take the ratio the global dirty
limits have to the global memory state and put it into proportion to
the individual zone. The allocator ensures that pages allocated for
being written to in the page cache are distributed across zones such
that there are always enough clean pages on a zone to begin with.
I am not yet really satisfied as it's not really orthogonal or
integrated with the other writeback throttling much, and has rough
edges here and there, but test results do look rather promising so
far:
--- Copying 8G to fuse-ntfs on USB stick in 4G machine
3.0:
Performance counter stats for 'dd if=/dev/zero of=zeroes bs=32k count=262144' (6 runs):
140,671,831 cache-misses # 4.923 M/sec ( +- 0.198% ) (scaled from 82.80%)
726,265,014 cache-references # 25.417 M/sec ( +- 1.104% ) (scaled from 83.06%)
144,092,383 branch-misses # 4.157 % ( +- 0.493% ) (scaled from 83.17%)
3,466,608,296 branches # 121.319 M/sec ( +- 0.421% ) (scaled from 67.89%)
17,882,351,343 instructions # 0.417 IPC ( +- 0.457% ) (scaled from 84.73%)
42,848,633,897 cycles # 1499.554 M/sec ( +- 0.604% ) (scaled from 83.08%)
236 page-faults # 0.000 M/sec ( +- 0.323% )
8,026 CPU-migrations # 0.000 M/sec ( +- 6.291% )
2,372,358 context-switches # 0.083 M/sec ( +- 0.003% )
28574.255540 task-clock-msecs # 0.031 CPUs ( +- 0.409% )
912.625436885 seconds time elapsed ( +- 3.851% )
nr_vmscan_write 667839
3.0-per-zone-dirty:
Performance counter stats for 'dd if=/dev/zero of=zeroes bs=32k count=262144' (6 runs):
140,791,501 cache-misses # 3.887 M/sec ( +- 0.186% ) (scaled from 83.09%)
816,474,193 cache-references # 22.540 M/sec ( +- 0.923% ) (scaled from 83.16%)
154,500,577 branch-misses # 4.302 % ( +- 0.495% ) (scaled from 83.15%)
3,591,344,338 branches # 99.143 M/sec ( +- 0.402% ) (scaled from 67.32%)
18,713,190,183 instructions # 0.338 IPC ( +- 0.448% ) (scaled from 83.96%)
55,285,320,107 cycles # 1526.208 M/sec ( +- 0.588% ) (scaled from 83.28%)
237 page-faults # 0.000 M/sec ( +- 0.302% )
28,028 CPU-migrations # 0.001 M/sec ( +- 3.070% )
2,369,897 context-switches # 0.065 M/sec ( +- 0.006% )
36223.970238 task-clock-msecs # 0.060 CPUs ( +- 1.062% )
605.909769823 seconds time elapsed ( +- 0.783% )
nr_vmscan_write 0
That's an increase of throughput by 30% and no writeback interference
from reclaim.
As not every other allocation has to reclaim from a Normal zone full
of dirty pages anymore, the patched kernel is also more responsive in
general during the copy.
I am also running fs_mark on XFS on a 2G machine, but the final
results are not in yet. The preliminary results appear to be in this
ballpark:
--- fs_mark -d fsmark-one -d fsmark-two -D 100 -N 150 -n 150 -L 25 -t 1 -S 0 -s $((10 << 20))
3.0:
real 20m43.901s
user 0m8.988s
sys 0m58.227s
nr_vmscan_write 3347
3.0-per-zone-dirty:
real 20m8.012s
user 0m8.862s
sys 1m2.585s
nr_vmscan_write 161
Patch #1 is more or less an unrelated fix that subsequent patches
depend upon as they modify the same code. It should go upstream
immediately, me thinks.
#2 and #3 are boring cleanup, guess they can go in right away as well.
#4 adds per-zone dirty throttling for __GFP_WRITE allocators, #5
passes __GFP_WRITE from the grab_cache_page* functions in the hope to
get most writers and no readers; I haven't checked all sites yet.
Discuss! :-)
include/linux/gfp.h | 4 +-
include/linux/pagemap.h | 6 +-
include/linux/writeback.h | 5 +-
mm/filemap.c | 8 +-
mm/page-writeback.c | 225 ++++++++++++++++++++++++++++++--------------
mm/page_alloc.c | 27 ++++++
6 files changed, 196 insertions(+), 79 deletions(-)
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