Evaluation and Enhancement to Memory Sharing and Swapping in Xen 4.1

Evaluation and Enhancement to Memory
Sharing and Swapping in Xen 4.1
Xiaowei Yang, Chuan Ye, Qiangmin Lin
{xiaowei.yang, yechuan, linqiangmin}@huawei.com
HUAWEI
Sponsored by:

&

Agenda
• Background
• Memory Overcommit Features / Policy
• Evaluation

Background
• Challenges
• Memory becomes bottleneck as VM# grows
• E.g.: VM# > 100 in VDI scenario on 2-socket server
• What’s the proper vMem size?
• vMem# too small: bad performance
• vMem# too big: low utilization

• Memory overcommit is a key factor to high VM density /
memory utilization

• ESX / KVM have rich memory overcommit features / policies
• ESX: Balloon, TPS, Host swap, compression
• KVM: Balloon, KSM, Host swap

• Xen adds sharing / swapping since 4.0, but
• Untested in production
• Need improvements in terms of efficiency, performance
• No policies

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential

Goals
• Evaluate current memory overcommit features in Xen
4.1, particularly memory sharing / swapping

• Make enhancements to current memory sharing /
swapping features

• Design a memory overcommit policy to reach higher
VM density w/o sacrificing performance


Blktap2 Sharing
How it works
Rely on link-clone of parent image

Share
1st read:
• tapdisk2 read from the parent
image on disk, and records the
read / page (p1)

Later read:
• Tapdisk2 finds the record,
notifies Xen HV to share with p1

Unshare
• Write to the RO sharing page
triggers unshare


Blktap2 Sharing (2)
• Pros
• Overhead is lower than content-based page sharing
• No need to calculate/compare page contents in the background
• Relief VM bootup storm issue
• Later read from memory cache

• Cons
• Sharing% is much lower comparing to content-based sharing
• Only VMs with the same parent image can share pages
• Rely on PV driver
• No share before PV driver loaded – startup
• Page sharing # is limited by shm #
• Between tapdisk2 processes to store global hash table


0-page Sharing
How it works

Share
• zerosharing triggers Xen HV to
scan VM’s page contents
periodically
• If Xen HV finds the page’s
content is all 0, free the original
page, and points VM’s
corresponding p2m entry to a
special RO 0-page

Unshare
• Write to the RO 0-page triggers
unshare

* Red words are our enhancements

0-page Sharing (2)
• 0-page sharing is the most valuable part in content-
based page sharing per our evaluation

• 0-page sharing is more useful to Windows VM than to
Linux VM
• All free mem are 0-page in Windows – it scrubs before free

• A proper scan rate of 0-page sharing is important
• Slow: can’t relief memory pressure in time
• Fast: high CPU util%

• Actually 0-page sweep is used in POD
• Usage limited - only when POD cache under the pressure


Host Swap
How it works

Swap out
• xenpaging selects guest pages
thru policy
• Xenpaing saves their contents to
disk, then notifies Xen HV to free
them

Swap in
• Guest access to the swapped-out
page triggers violation
• Xen HV notifies xenpaging to
read back the contents to a new
allocated page


Host Swap (2)
• Pros
• The only memory overcommit feature that guarantees
overcommit ratio

• Cons
• Page selection is hard
• Which page is proper to be swapped out?
• Inefficient
• Disk access latency is much higher
• Double swap scenario


Host Swap Policies
• Basics
• Random
• Sequential

• Improvements
• Skip low memory: which is used for BIOS, kernel image
• MRU: prevent recent swapped-in pages from being swapped out
• Aggressive MRU: prevent X continuous pages adjacent to each
MRU page from being swapped out
• Swap->sharing: If the elected page is 0-page, share it instead of
swap

• Advanced
• Based on statistic of Guest OS page usage in HV
• Based on Guest OS MM knowledge – enlightened


Memory Overcommit Policy
• Metrics
• Host Free Memory #
• VM Free Memory #
• VM Maximum Memory
• VM Reserved Memory
• VM Current Zero Page #
• VM Current Balloon Page #
• VM Current Swap Page #
• VM Current Sharing Page #

• Configure options
• Sharing threshold (default: 20%)
• Balloon threshold (default: 10%)
• Swap threshold (default: 5%)
• 0-page sharing scan rate (frequency, page #)
• …


Memory Overcommit Policy (2)
• No memory pressure
• Turn off memory overcommit used% < 80%

• Host memory pressure is moderate
• Turn on blktap2 / 0-page sharing
used% > 80%
• Set 0-page sharing scan rate

• Host memory pressure is severe
• Adjust 0-page sharing scan rate used% > 90%
• Start balloon; balloon # by VM metrics

• Host memory pressure is critical
• Balloon can’t afford memory consumption used% > 95%
• Start swap

• When the pressure goes down, return memory to VM


Memory Overcommit Support Matrix
Balloon Sharing Host Swap
QEMU OK Sharing Trigger swap in
breaks
PV driver OK Sharing Trigger swap in
breaks
I/O device passthru OK Conflict! Conflict!
VM live migration OK Sharing 1. Trigger swap in
breaks 2. Swapfile
accessible after L.M.
VM save OK OK Triggers swap in
vMem snapshot OK OK Triggers swap in
VM resume OK Sharing OK
breaks
VM hibernate 1. Balloon in OK Triggers swap in
2. Redirect to
0-page


Experimental Environment
Host Configure
Processor 2x Intel X5670 @ 2.93GHz, SMT enabled
Memory 96GB DDR3
Storage Intel 160G X25-M SSD

VM Workloads
SPECjbb 1 vCPU, 4G vMem; SLES 11
Heap size: 2.5GB
Kernel Build 1 vCPU, 512M vMem; SLES 11
Linux kernel: 2.6.32
Sysbench OLTP 1 vCPU, 1G vMem; SLES 11
Database: mysql
VDI benchmark 1 vCPU, 1G vMem; Windows 7
Workload: Office, IE, PDF, Java


Blktap2 / 0-page Sharing # -- VM startup

Unshared #

• 0-page sharing # is dominant
• Xen scrubs pages (on host startup, domain destroy, …)
• Lots of `free memory’ are 0-page, can be shared

• Linux uses more memory to boot up
• Less 0-page sharing #
• More blktap2 sharing #


Blktap2 / 0-page Sharing # -- VDI workload

VM’s Sharing # diff w/ VDI workload
Before (MB) After (MB) Diff
0-page sharing # 750 400 ↓53%
Blktap2 sharing # 42 171 ↑409%
Unshared # 199 418 ↑210%


0-page Sharing # -- Windows v.s. Linux

• On startup almost all 0-pages are from `free memory’
• Windows: Free memory is 0-page all the time
• Windows scrubs page before free
• More friendly to page sharing
• Linux: Free memory is 0-page only on startup
• Linux doesn’t scrub free page

* Mem Hog test case consumes 500MB memory when running

Performance Impact of blktap2 Sharing

• Performance impact of blktap2 is negligible
• Scalability is very good

• In theory blktap2 sharing could benefit READ intensive workload
• First time: from disk; afterwards: from cache
• But the benefit is not observed in KB test


Performance Impact of 0-page Sharing

• Few impacts on the benchmarks’ scores
• Impacts of different scan rates are almost the same
• 5%-7% CPU overhead in dom0
• Few new 0-pages are generated during the test – scan finishes fast
• A better benchmark?


Host Swap Policies

• Different policies result in different performances
• Swap->sharing policy brings the best performance most of the
time
• When the remaining vMem# < working set, the performance
drops dramatically


Host Swap v.s. Balloon

• Balloon usually performs better than swap
• Balloon transfers the memory pressure from host to guest
• Guest OS knows better about memory usage: which is free; which is
not least/most used

• Swap->sharing policy narrows the gap between swap / balloon


VM Density -- VDI Workload
Projected

* Host memory = 96GB

1. W/ no memory overcommit: VM# = 85, memory is bottleneck
2. W/ balloon: VM# = 120, CPU/memory are both bottlenecks
3. W/ balloon+sharing: when VM# = 120, host free memory = 17GB,
CPU is bottleneck
4. W/ balloon+sharing: w/o CPU bottleneck, same memory can host
145 VM (projected)
* Test 1/2/3 are performed w/ QoE unchanged

Takeaways
• 0-page sharing complements blktap2 sharing. The
combination of both is competitive

• The performance impact and overhead of blktap2 / 0-
page sharing is small if used properly

• Windows is more friendly to 0-page sharing than Linux

• Host swap policy do matters. Swap->sharing policy
narrows the gap between swap / balloon

• In VDI scenario a good memory overcommit policy can
increase VM density by 70+% w/ QoE uncompromised


Evaluation and Enhancement to Memory Sharing and Swapping in Xen 4.1

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