an accurate prefetch technique for dynamic paging behaviour for software distributed shared memory

Jie Cai and Peter Strazdins

Research School of Computer Science

The Australian National University

ICPP 2012

Pittsburgh, PA, USA

An Accurate Prefetch Technique for Dynamic Paging Behaviour for Software Distributed Shared

Memory

• Introduction• Background• Related Work on Existing Prefetch Techniques• Stride-augmented Run-length Encoding Method

(sRLE)• Dynamic Region-based Prefetch Technique• Evaluation Results• Conclusion

Outline

ICPP 2012 @ Pittsburgh, PA

• Software Distributed Shared Memory (sDSM) systems provide programming environments that enable the use of shared programming model such as OpenMP on clusters.

• sDSM systems inherit the good programmability of shared memory programming models. • Removing explicit control of data exchange from programmer

• However, sDSM suffers from significant system overheads.• Prefetch techniques, fitting well with lazy release consistency (LRC), can

be used to improve performance.

• Prefetch techniques for sDSM face two major challenges:• Applications’ dynamic memory access patterns• Page misses caused by non-global synchronization operations

Introduction


• In this talk, we address the challenges of prefetch techniques for sDSM systems• Reconstruct page miss record using strided-augmented

run-length encoding (sRLE) method• Designed a dynamic region-based prefetch (DReP)

technique based on sRLE’d records to predict and issue prefetches.

• Implemented into the only commercialized sDSM system, Intel Cluster OpenMP (CLOMP)

• DReP and sRLE with CLOMP are evaluated using NPB-OMP benchmark suite, LINPACK, and a memory consistency cost micro-benchmark (MCBENCH)

Introduction (Cont.)


Background (1)


Start parallel region, implicit barrier

Single Thread Sequential Region #3



Parallel Region #1Thread1 Thread2 Thread3Thread0

Explicit barrier

Thread1 Thread2 Thread3Thread0 Parallel Region #2

End parallel region, implicit barrier








Parallel Region #1Thread1 Thread2 Thread3Thread0

Explicit barrier






• Fork-join type shared memory programming models:• Regions are separated

using global synchronizations, e.g. implicit and explicit barriers;

• Region-executions are multiple executions of the same region when this region is enclosed in a loop.

Background (2)


• sDSM memory consistency model• Each process has a local

view of the shared pages• The shared pages are

kept consistent via mprotect (please refer to the page state machine for details).

Local

Memory

Local

Memory… … …MPI

Shared Memory Programming Model (OpenMP)

Virtual Shared Memory

Global memory is managed in blocks, pages

Local

Memory

Local

Memory… … …MPI

Shared Memory Programming Model (OpenMP)

Virtual Shared Memory

Global memory is managed in blocks, pages

Background (3)


• sDSM memory consistency costs• The major sDSM system

overhead is the memory consistency cost.

• MCBENCH is a in-house developed micro-benchmark that measures this cost for different OpenMP implementations, including cluster enabled OpenMPs.

Related Work


• Dynamic Aggregation (C. Amza et al. 1997)• Simple assumption of temporal paging behavior before and after

a barrier.

• B+ and Adaptive++ (R. Bianchini et al. 1996 & 1998)• B+: simple assumption of temporal paging behavior before and

after a barrier.• Adaptive++: assuming page misses occurred before a barrier or

even before the previous barrier will occur again after the barrier.

• Third order differential finite context method (TODFCM) (E. Speight et al. 2002)• Generic technique prefetch a page when three previous

consecutive misses had happened before.

Related Work (Cont.)


• Temporal region-based pretech (TReP) technique (J. Cai et al. 2010)• Deployed idea of region and region-executions• Assume page misses in the previous region-execution will occur

in the current region-execution• Considered temporal paging behaviour between consecutive

region-executions

• Hybrid region-based prefetch (HReP) technique (J. Cai et al. 2010)• Deployed idea of region and region-executions• Combined TReP and Adaptive++

• Addressed temporal paging behaviour between consecutive region-executions and spatial paging behaviour within a region-execution.

sRLE Method -- Observation


• LINPACK dynamic page access pattern with 4 processes

• Corresponding dynamic page miss pattern

sRLE Method


• Step (a) group sub-list with common stride;

• Step (b) encode the sub-lists into first level format: • (start page, stride, run

length)

• Step (c) group consecutive encoded sub-lists with common stride into second level encoding format:• (first level encoded record,

stride, run length)• Ordinary page fault list can

be converted to 2D fault regions with sRLE.

DReP Technique Designs


• All page fault records (per region) has been encoded twice with sRLE method.

• Each record contains a list of second level encoded entries.

DReP Technique Designs (cont.)


• At the beginning of each region-execution, DReP predict and prefetch pages.

Beginning of a region-execution

Previouslyexecuted twice?

No

No Prefetch issued

Yes

Compare every entries between two records. Issue prefetches ONLY for

the following three cases.

Prefetch the entry if it is common to both

lists

When strides and run lengthes are common to both lists, predict a start page, and prefetch with the common

strides and run length

pred.l1_en_col.start_page = p_list.l1_en_col.start_page +(p_list.l1_en_col.start_page − bp_list.l1_en_col.start_page)

When strides are common and run lengthes are highly similar to both lists, predict a start page and a run lengthes, then prefetch with the common strides.

Case 1: Case 2: Case 3:

pred.l1_en_col.run len = p_list.l1_en_col.run_len +(p_list.l1_en_col.run_len − bp_list.l1_en_col.run_len)

pred.run_len = p_list.run_len + (p_list.run_len−bp_list.run_len)

DReP Implementation


• DReP has been implemented into Intel Cluster OpenMP runtime.• New region notification user interactive interface:

• KMP_USER_NOTIFY_NEW_REGION(1) : 1 indicates this is a parallel region• KMP_USER_NOTIFY_NEW_REGION(0): 0 indicates this is a sequential region

• Flush filtering solved the problem of single page can be missed multiple times within one region-execution by removing duplicated records.

• Enlarged message header of the communication layer which can accommodate 128 page IDs to leverage network bandwidth.

• Each process first communicate to its right neighbor that avoid network congestion.

DReP Implementation (Cont.)


• DReP has been implemented into Intel Cluster OpenMP runtime.• Page state machine

has been updated with two new introduced page states

• Prefetched_diff• Prefetched_page

Evaluation


• Experimental setup• Software and benchmarks

• NPB-OMP suite• LINPAK OpenMP implementation (n=8196, nb=64)• MCBENCH (a = 4MB, c = 4B and 4KB)

• Hardware platform• 8-node Intel cluster• Each node consists of 2 Intel E5472 3.0Ghz CPUs• 16GB memory• Gigabit Ethernet• DDR Infiniband

Efficiency and Coverage


• Nf: total number of page faults• Np: number of prefetches• Nu: number of useful prefetches, Nu = Nf*C• C = Nu/Nf, coverage• E = Nu/Np, efficiency• Bold font represents best results

Efficiency and Coverage (Cont.)


• Bold font represents best results

• MCBENCH: DReP vs TReP and HReP• c = 4B: extreme false sharing• c= = 4KB: no false sharing

Memory Consistency Cost


• Measured using MCBENCH, a = 4MB, c = 4B and 4KB• c = 4B: extreme false sharing (reduced ~86% cost)• c = 4KB: no false sharing

Memory Consistency Cost (Cont.)


• LINPACK OpenMP implementation with n=8196 and nb=64• DReP is represented as a reduction rate to that of original

CLOMP implementation, e.g. (Orig-DReP)/Orig.

Memory Consistency Cost (Cont.)


• NPB-OMP• Rates are represented as an average of each class from A to

C.

Overhead Analysis of DReP


• NPB-OMP IS.C• Tsegv: total memory consistency cost in seconds for original CLOMP and

DReP enabled CLOMP.• TMK Comm (% to Tsegv): communication time spent in the DSM layer of

CLOMP (TMK)• TMK local (% to Tsegv): the local software overhead of TMK layer• DReP Comm (% to Tsegv): communication cost of data prefetching• DReP local (% to Tsegv): the local software cost introduced by DReP• Communication costs are further broke down to cost for transferring diffs and

pages.

Conclusions


• With assistance of sRLE, DReP accurately analyses the paging behaviour exhibiting both static and dynamic memory access patterns, such as NPB-OMP and LINPACK.

• On average of NPB and LINPACK, DReP improves 34% efficiency and 47% coverage based on existing prefetch techniques, in details:

• 55% and 5% better efficiency compared to Adaptive++ and TODFCM; 55% and 44% better coverage compared to Adaptive++ and TODFCM

• 47% and 30% better efficiency compared to TReP and HReP; and 56% and 34% better coverage compared to TReP and HReP.

• DReP dramatically reduces 86% memory consistency cost for the false sharing scenario; and ~45% and ~38% for LINPACK and NPB on GigE and IB respectively.

• A detailed breakdown analysis showed a ~2% introduced overhead for DReP.

Acknowledgement


• Australian Research Council Grant LP0669726• ANU CECS Faculty Research Grant• Intel Corp.• Sun Microsystems• NCI National Facility / ANU Supercomputer Facility

an accurate prefetch technique for dynamic paging behaviour for software distributed shared memory

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