On 20/01/14 09:54, Christian Ehrhardt wrote:
On 20/01/14 02:05, Shaohua Li wrote:
On Fri, Jan 17, 2014 at 01:39:43PM +0100, Christian Ehrhardt wrote:
[...]
Is the swap disk a SSD? If not, there is no point to partition the
disk. Do you
see any changes in iostat in the bad/good case, for example, request
size,
iodepth?
Hi,
I use normal disks and SSDs or even the special s390 ramdisks - I agree
that partitioning makes no sense in a real case, but it doesn't matter
atm. I just partition to better show the effect that "more swap targets
-> less throughput" - and partitioning makes it easy for me to guarantee
that the HW ressources serving that I/O stay the same.
IOstat and such things don't report very significant changes regarding
I/O depth. Sizes are more interesting with the bad case having slightly
more (16%) read I/Os and dropping average request size from 14.62 to
11.89. Along with that goes a drop in read request merges of 28%.
But I don't see how a workload that is random in memory would create
significantly better/worse chances for request merging depending on the
case if the disk is partitioned more or less often.
On the read path swap doesn't care about iterating disks, it just goes
by associated swap extends -> offsets to the disk.
And I thought in a random load that should be purely random and hit each
partition in e.g. the 4 partition case just by 25%.
I checked some blocktraces I had and can confirm as expected each got an
equal share.
There is one patch can avoid swapin reads more than swapout for random
case,
but still not in upstream yet. You can try it here:
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/mm/swap_state.c?id=5d19b04a2dae73382fb607f16e2acfb594d1c63f
Great suggestion - it sounds very interesting to me, I'll give it a try
in a few days since I'm out Tue/Wed.
I had already a patch prepared and successfully tested that allows to
configure page cluster for read/write separately from userspace. That
worked well but would require an admin to configure the "right" value
for his system.
Since that fails with so much kernel tunables and would also not be
adaptive if the behaviour changes over time I very much prefer your
solution to it.
That is why I tried to verify your patch in my environment with at least
some of the cases I used recently for swap analysis and improvement.
The environment has 10G of real memory and drives a working set of 12.5Gb.
So just a slight 1.25:1 overcommit (While s390 often runs in higher
overcommits for most of the linux Swap issues so far 1.25:1 was enough
to trigger and produced more reliable results).
To swap I use 8x16G xpram devices which one can imagine as SSDs at main
memory speed (good to make forecasts how ssds might behave in a few years).
I compared a 3.10 kernel (I know a bit old already, but I knew that my
env works fine with it) with and without the patch for swap readahead
scaling.
All memory is initially completely faulted in (memset) and thne warmed
up with two full sweeps of the entire working set following the current
workload configuration.
The unit reported is MB/s the workload can achieve in its
(overcommitted) memory being an average of 2 runs for 5 minutes each (+
the init and warmup as described).
(Noise is usually ~+/-5%, maybe a bit more in non exclusive runs like
this when other things are on the machine)
Memory Access is done via memcpy in either direction (R/W) with
alternating sizes of:
5% 65536 bytes
5% 8192 bytes
90% 4096 bytes
Further abbreviations
PC = the currently configured page cluster size (0,3,5)
M - Multi threaded (=32)
S - Single threaded
Seq/Rnd - Sequential/Random
No Swap RA With Swap RA Diff
PC0-M-Rnd ~= 10732.97 9891.87 -7.84%
PC0-M-Seq ~= 10780.56 10587.76 -1.79%
PC0-S-Rnd ~= 2010.47 2067.51 2.84%
PC0-S-Seq ~= 1783.74 1834.28 2.83%
PC3-M-Rnd ~= 10745.19 10990.90 2.29%
PC3-M-Seq ~= 11792.67 11107.79 -5.81%
PC3-S-Rnd ~= 1301.28 2017.61 55.05%
PC3-S-Seq ~= 1664.40 1637.72 -1.60%
PC5-M-Rnd ~= 7568.56 10733.60 41.82%
PC5-M-Seq ~= n/a 11208.40 n/a
PC5-S-Rnd ~= 608.48 2052.17 237.26%
PC5-S-Seq ~= 1604.97 1685.65 5.03%
(for and PC5-M-Seq I ran out of time, but the remaining results are
interesting enough already)
I like what I see, there is nothing significantly out of the noise range
which shouldn't be.
The Page Cluster 0 cases didn't show an effect as expected.
For page cluster 3 the multithreaded cases have hidden the impact to TP
due to the fact that then just another thread can continue.
But I checked sar data and see that PC3-M-Rnd has avoided about 50% of
swapins while staying at equal throughput (1000k vs 500k pswpin/s).
Other than that Random loads had the biggest improvements matching what
I had with splitting up read/write page-cluster size.
Eventually with page cluster 5 even the multi threaded cases start to
show benefits of the readahead scaling code.
In all that time sequential cases didn't change a lot.
So I think that test worked fine. I see there were some discussion son
the form of the implementation, but in terms of results I really like it
as far as I had time to check it out.
*** Context switch ***
Now back to my original question about why swapping to multiple targets
makes things slower.
Your patch helps there a but as the Workload with the biggest issue was
a random workload and I knew that with pagecluster set to zero the loss
of efficiency with those multiple swap targets is stopped.
But I consider that only a fix of the symptom and would love if one
comes up with an idea actually *why* things get worse with more swap
targets.
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