SUPERPAGES

Practical ,Transparent Operating System Support For

Authors:

Juan Navarro, Sitaram Iyer, Peter Drusche, Alan Cox

Presented by:

Nadeeshani Hewage

Basics & Terminology

• TLB?

o Caches virtual-to-physical address mappings from the page tables

o Faster access

• TLB Coverage?

o Amount of memory accessible through cached mappings, without TLB

misses

• Superpage

o Memory page for larger size than a Base-page (ordinary memory page)

o Multiple sizes

o Single entry in TLB per superpage

• Memory objects

o Occupy some contiguous space in virtual memory

o Eg: memory mapped files, code, data

• TLB coverage decreasing with growing memory size over years

• Motivation to increase TLB coverage without enlargening TLB size.

Constraints for Superpages

• Page size should be supported by processor.

• Contiguous

• Starting address in physical & virtual address space must be a multiple of its

size.

• Single TLB entry for a superpage. Meaning single set of protection

attributes and single dirty bit for a superpage.

Considerations

• Allocation

o Allocating physical frames for the memory object.

o If there exists a plan to create a superpage by the OS, to satisfy the

contiguity requirement,

Relocation based allocation

• Create contiguity

• Physically copying allocated page frames when it is decided

that superpages are likely to be beneficial

o Software managed TLB + TLB miss handler

o Counter per superpage incremented at TLB miss

Reservation based allocation

• Preserve contiguity

• Make allocation decisions at page fault time

• Decide preferred size of allocation on an algorithm

o Eg: Talluri & Hill proposed

• Fragmentation Control

o Release contiguous chunks of inactive memory from previous

allocations

o Preempt an existing partially used reservation

• Promotion

o Set of base pages from a potential super page satisfying constraints

promoted to a Superpage.

• Demotion

o Reduce size of a superpage

A smaller super page

Base pages

• Eviction

o At a demand of memory preassure, inactive superpage is evicted from

physical memory.

Design & Implementation

Preferred Superpage Size Policy

o Decision made early at process execution

o Looks only at attributes of memory object

o Decided size

Too large: decision overridden by preempting init reservation

Too small: cannot be reverted

o Policy: Pick the maximum superpage size that can be used for an object

Reservation based Allocation

• Already spoken

Preempting Reservations

• Free physical memory scarce/ excessively fragmented

• Preempt reserved but unused frames

• Choice between

o Refusing allocation; reserving smaller extent than desired for new

allocation

o Preempting existing reservation which has enough unallocated frames

• Policy: Whenever possible preempt & give space for alloaction

Incremental Promotions, Demotions

• Promotions

o Multiple superpage sizes exist

o Superpage gets created when any size supported gets fully populated

within a reservation

o Increment smaller superpage to next larger size superpage

• Demotions

o A base page of a superpage targetted to be evicted by page daemon,

causes a demotion

o Demote to next smaller superpage size

Paging Out Dirty Superpages

• OS keeps only single dirty bit for the whole superpage.

• Problem: only one base page modified; cost of writing full superpage to disk

(high cost on I/O)

• Practice: Demote clean superpages whenever process attempts to write to

them. Repromote later.

Reservation Lists

• Keep track of reserved page extent that are not fully populated

• One list per each page size supported.

• Kept sorted by the time of their most recent page frame allocation

• Policy: Preempt the extent whose most recent allocation, occurred least

recently among all reservations in the list (head)

Population Map

• Keep track of allocated base pages within each memory object

• Queried on every page fault, should support efficient lookups.

• Radix tree

o Each level corresponds to a page size

o Root: max superpage size supported

o Non-leaf:

# of supperpage sized virtual regions at next lower level with a

population of at least one (somepop)

# of supperpage sized virtual regions at next lower level fully

populated (fullpop)

• Used for various decisions

Wired Page clustering

• Pages used for internal datastructures for the OS (implementation on

FreeBSD) are wired

(not evicted)

• At system boot time, clustered together in physical memory.

• Gradually as kernel allocates memory they get scattered, causes

fragmentation

• To avoid fragmentation, identify pages which are to be wired (internal use),

cluster them in pools of contiguous physical memory

Evaluation

Platform & Workload

• Platform

o FreeBSD 4.3 kernel as a loadable module

o Alpha 21264 processor at 500 MHz

o four page sizes: 8KB base pages, 64KB, 512KB and 4MB superpages

o 512MB RAM

o 64KB data and 64KB instruction L1 caches, virtually indexed and 2-way

associative

o fully associative TLB with 128 entries for data and 128 for instructions

• Workload

o FFTW: The Fastest Fourier Transform in the West with a 200x200x200

matrix as input

o Matrix: A non-blocked matrix transposition of a1000x1000 matrix

o Web: The thttpd web server servicing 50000 requests selected from an

access log of the CS departmental web server at Rice University. The

working set size of this trace is 238MB, while its data set is 3.6GB.

Best case benefits

• When free memory is plentiful

& non fragmented

o Speedup

(mean elapsed runtime of 3

runs after an initial warmup)

o TLB misses reduction

percentage

Evaluation ctd.

• Fragment system by running web server & feeding it with requests

• FFTW run on fragmented system 4 times in sequence.

• Goal : To see how quickly the system recovers just enough contiguous

memory to build superpages

• 2 contiguity restoration techniques

o Cache scheme

Treat all cache pages as available, coalesces them into allocator

0 superpages

o Daemon scheme

Contiguity aware page replacement + wired page clustering

Get 20/60, 35/60, 38/60, 40/60 superpages

• Even if the superpages implementation is there

it requires a promising fragmentation control

mechanism to acquire promising results.

• With superpages and effective fragmentation control, evaluations have

given promising results.

Thank You!