M5 simulator system TDT4260 Computer Architecture User
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1. Since programs often do some initialization and setup on startup a sample from the start of a program run is unlikely to be representative for the whole program It is therefore desirable to begin the performance tests after the program has been running for some time To save simulation time M5 can resume a program state from a previously stored checkpoint The prefetcher framework comes with checkpoints for the CPU2000 benchmarks taken after 10 instructions It is often useful to run a specific test to reproduce a bug To run the CPU2000 tests outside of test prefetcher py you will need to set the M5 CPU2000 environment variable If this is set incorrectly M5 will give the error message Unable to find workload To export this as a shell variable do export M5 CPU2000 1ib cpu2000 Near the top of test prefetcher py there is a commented out call to dry run If this is uncommented test prefetcher py will print the command line it would use to run each test This will typically look like this m5 build ALPHA SE m5 opt remote gdb port 0 re outdir output ammp user mb configs example se py checkpoint dir lib cp checkpoint restore 1000000000 at instruction caches 12cache standard switch warmup insts 10000000 max inst 10000000 12size 1MB bench ammp prefetcher on access true policy proxy This uses some additional command line options these are explained in Table 2 2 Option Description bench ammp2. 1 1 Overview This documentation covers the following e Installing and running the simulator e Machine model and memory hierarchy e Prefetcher interface specification e Using the interface e Testing and debugging the prefetcher on your local machine e Submitting the prefetcher for benchmarking e Statistics 1 2 Chapter outlines The first chapter gives a short introduction and contains an outline of the documentation The second chapter starts with the basics how to install the M5 simulator There are two possible ways to install and use it The first is as a stand alone VirtualBox disk image which requires the installation of VirtualBox This is the best option for those who use Windows as their operating system of choice For Linux enthusiasts there is also the option of downloading a tarball and installing a few required software packages The chapter then continues to walk you through the necessary steps to get M5 up and running building from source running with command line options that enables prefetching running local benchmarks compiling and running custom test programs and finally how to submit your prefetcher for testing on a computing cluster The third chapter gives an overview over the simulated system and de scribes its memory model There is also a detailed specification of the prefetcher interface and tips on how to use it when writing your own prefetcher It includes a very simple example pref
3. Run one of the SPEC CPU2000 benchmarks checkpoint dir lib cp The directory where program checkpoints are stored at instruction Restore at an instruction count checkpoint restore n The instruction count to restore at standard switch Warm up caches with a simple CPU model then switch to an advanced model to gather statistics warmup insts n Number of instructions to run warmup for max inst n Exit after running this number of instructions Table 2 2 Advanced se py command line options 2 4 2 Running M5 with custom test programs If you wish to run your self written test programs with M5 it is necessary to cross compile them for the Alpha architecture The easiest way to achieve this is to download the precompiled compiler binaries provided by crosstool from the M5 website Install the one that fits your host machine best 32 or 64 bit version When cross compiling your test program you must use the static option to enforce static linkage To run the cross compiled Alpha binary with M5 pass it to the script with the cmd option Example build ALPHA SE mb5 opt configs example se py detailed caches l2cache 12size 512kB prefetcher policy proxy prefetcher on access True cmd path to testprogram 2 5 Submitting the prefetcher for benchmarking First of all you need a user account on the PfJudge web pages The teaching assistant in TDT4260 Computer Architecture will create one for you You must also b
4. it reports any potential problems By default M5 uses a custom memory allocator instead of malloc This will not work with Valgrind since it replaces malloc with its own custom mem ory allocator Fortunately M5 can be recompiled with NO_FAST_ALLOC True to use normal malloc scons NO FAST ALLOC True mb build ALPHA SE mb debug To avoid spurious warnings by Valgrind it can be fed a file with warning suppressions To run M5 under Valgrind use valgrind suppressions lib valgrind suppressions m5 build ALPHA_SE m5 debug Note that everything runs much slower under Valgrind 18
5. on some or all tests status will be Runtime error To locate the failed tests check the detailed view You can take a look at the output from the failed tests by clicking on the output link found after each test statistic To allow easier exploration of different prefetcher configurations it is possi ble to submit several prefetchers at once bundled into a zipped file Each cc file in the archive is submitted independently for testing on the cluster The submission is named after the compressed source file possibly prefixed with the name specified in the submission form There is a limit of 50 prefetchers per archive Chapter 3 The prefetcher interface 3 1 Memory model The simulated architecture is loosely based on the DEC Alpha Tsunami system specifically the Alpha 21264 microprocessor This is a superscalar out of order OoO CPU which can reorder a large number of instructions and do speculative execution The L1 prefetcher is split in a 32kB instruction cache and a 64kB data cache Each cache block is 64B The L2 cache size is 1MB also with a cache block size of 64B The L2 prefetcher is notified on every access to the L2 cache both hits and misses There is no prefetching for the L1 cache The memory bus runs at 400MHz is 64 bits wide and has a latency of 30ns 3 2 Interface specification The interface the prefetcher will use is defined in a header file located at prefetcher interface hh To use th
6. request queue The cache will issue requests from the queue when it is not fetching data for the CPU This queue has a fixed size available as MAX QUEUE SIZE and when it gets full the oldest entry is evicted If you want to check the current size of this queue use the function current queue size void 3 3 Using the interface Start by studying interface hh This is the only M5 specific header file you need to include in your header file You might want to include standard header files for things like printing debug information and memory alloca tion Have a look at what the supplied example prefetcher a very simple sequential prefetcher to see what it does If your prefetcher needs to initialize something prefetch init is the place to do so If not just leave the implementation empty You will need to implement the prefetch access function which the cache calls when accessed by the CPU This function takes an argument AccessStat stat which supplies information from the cache the address of the executing instruction that accessed cache what memory address was access the cycle tick number and whether the access was a cache miss The block size is available as BLOCK SIZE Note that you probably will not need all of this information for a specific prefetching algorithm If your algorithm decides to issue a prefetch request it must call the issue prefetch function with the address to prefetch from as argument The cache block c
7. M5 simulator system TDT4260 Computer Architecture User documentation Last modified January 20 2014 Contents 1 Introduction LL Overview 20g ospa urie Ro Ree REDE WOW Soe amp Oe Bob es 1 2 Chapter outlines 2059222992 m eee ee B Rx es 2 Installing and running M5 2l Dovnload s esera ee ene ete Roy qox e OR ny vus 2 2 Installation 2224292259 Ra xvm esos ko o SL CLA 4n o bk eee ce bee BO hos 2 2 3 VirtualBox disk image Z4 BUN es we eed woe ok es Hoe ee Be Oe ee ee es 2A RUD ge a a ew ae ee Ok Be Ee a 2 4 1 CPU2000 benchmark tests 2 4 2 Running M5 with custom test programs 2 5 Submitting the prefetcher for benchmarking 3 The prefetcher interface al Mem ry model 5 22224 o mm Rm c3 ee RO 3 2 Interface specification 2l ttot iraa oo Using ihe interface gt lt cser 9 om X eom o e 3 3 1 Example prefetcher 4 Statistics 5 Debugging the prefetcher 5 1 mb5 debug and trace flags 0 Dh GDB sao hoe ee Pe ee ee BI ER wk ewe ba Walep ud o dedere lo eae xoxo x94 be kA E onum ed Chapter 1 Introduction You are now going to write your own hardware prefetcher using a modified version of M5 an open source hardware simulator system This modified version presents a simplified interface to M5 s cache allowing you to con centrate on a specific part of the memory hierarchy a prefetcher for the second level L2 cache
8. d prefetches C total prefetches Coverage How many of the potential candidates for prefetches were actu ally identified by the prefetcher good prefetches cov cache misses without prefetching Identified Number of prefetches generated and queued by the prefetcher 15 Issued Number of prefetches issued by the cache controller This can be significantly less than the number of identified prefetches due to duplicate prefetches already found in the prefetch queue duplicate prefetches found in the MSHR queue and prefetches dropped due to a full prefetch queue Misses Total number of L2 cache misses Degree of prefetching Number of blocks fetched from memory in a single prefetch request Harmonic mean A kind of average used to aggregate each benchmark speedup score into a final average speedup n n 1 Ei l1 yn I podes Oe 16 Chapter 5 Debugging the prefetcher 5 1 m5 debug and trace flags When debugging M5 it is best to use binaries built with debugging support m5 debug instead of the standard build m5 opt So let us start by recompiling M5 to be better suited to debugging scons j2 build ALPHA SE m5 debug To see in detail what s going on inside M5 one can specify enable trace flags which selectively enables output from specific parts of M5 The most useful flag when debugging a prefetcher is HWPrefetch Pass the option trace flags HWPrefetch to M5 build ALPHA SE m5 debug
9. e assigned to a group to submit prefetcher code or view earlier submissions Sign in with your username and password then click Submit prefetcher in the menu Select your prefetcher file and optionally give the submission a name This is the name that will be shown in the highscore list so choose with care If no name is given it defaults to the name of the uploaded file If you check Email on complete you will receive an email when the results are ready This could take some time depending on the cluster s current workload When you click Submit a job will be sent to the Kongull cluster which then compiles your prefetcher and runs it with a subset of the CPU2000 tests You are then shown the View submissions page with a list of all your submissions the most recent at the top When the prefetcher is uploaded the status is Uploaded As soon as it is sent to the cluster it changes to Compiling If it compiles successfully the status will be Running If your prefetcher does not compile status will be Compile error Check Compilation output found under the detailed view When the results are ready status will be Completed and a score will be given The highest scoring prefetcher for each group is listed on the highscore list found under Top prefetchers in the menu Click on the prefetcher name to go a more detailed view with per test output and statistics If the prefetcher crashes
10. e prefetcher interface you should include interface hh by putting the line include interface hh at the top of your source file define Value Description BLOCK SIZE 64 Size of cache blocks cache lines in bytes MAX QUEUE SIZE 100 Maximum number of pending prefetch requests MAX PHYS MEM SIZE 275 1 The largest possible physical memory address Table 3 1 Interface defines NOTE All interface functions that take an address as a parameter block align the address before issuing requests to the cache 10 Function Description void prefetch_init void Called before any memory access to let the prefetcher initialize its data structures void prefetch_access AccessStat stat Notifies the prefetcher about a cache access void prefetch complete Addr addr Notifies the prefetcher about a prefetch load that has just completed Table 3 2 Functions called by the simulator Function Description void issue_prefetch Addr addr int get prefetch bit Addr addr int set_prefetch_bit Addr addr int clear prefetch bit Addr addr int in cache Addr addr int in mshr queue Addr addr int current queue size void void DPRINTF trace format Called by the prefetcher to initiate a prefetch Is the prefetch bit set for addr Set the prefetch bit for addr Clear the prefetch bit for addr Is addr currently in the L2 cache Is there a prefetch request for addr in the MSHR miss status holding regis
11. etcher with extensive com ments The fourth chapter contains definitions of the statistics used to quantita tively measure prefetchers The fifth chapter gives details on how to debug prefetchers using advanced tools such as GDB and Valgrind and how to use trace flags to get detailed debug printouts Chapter 2 Installing and running M5 2 1 Download Download the modified M5 simulator from the PfJudge website 2 2 Installation 2 2 1 Linux Software requirements specific Debian Ubuntu packages mentioned in paren theses e 3 4 6 lt g lt 44 e Python and libpython gt 2 4 python and python dev e Scons gt 0 98 1 scons e SWIG gt 1 3 31 swig e zlib zlibig dev e m4 m4 To install all required packages in one go issue instructions to apt get sudo apt get install g python dev scons swig zlibig dev m4 The simulator framework comes packaged as a gzipped tarball Start the ad venture by unpacking with tar xvzf framework tar gz This will create a directory named framework 2 2 2 VirtualBox disk image If you do not have convenient access to a Linux machine you can download a virtual machine with M5 preconfigured You can run the virtual machine with VirtualBox which can be downloaded from http www virtualbox org The virtual machine is available as a zip archive from the PfJudge web site After unpacking the archive you can import the virtual machine into VirtualBox by selecting Impo
12. gnores requests to blocks already in the cache include interface hh void prefetch_init void Called before any calls to prefetch_access This is the place to initialize data structures DPRINTF HWPrefetch Initialized sequential on access prefetcher n void prefetch_access AccessStat stat pf_addr is now an address within the _next_ cache block Addr pf_addr stat mem_addr BLOCK_SIZE Issue a prefetch request if a demand miss occured and the block is not already in cache if stat miss amp amp in_cache pf_addr issue_prefetch pf_addr void prefetch complete Addr addr 1 Called when a block requested by the prefetcher has been loaded 14 Chapter 4 Statistics This chapter gives an overview of the statistics by which your prefetcher is measured and ranked IPC instructions per cycle Since we are using a superscalar architecture IPC rates gt 1 is possible Speedup Speedup is a commonly used proxy for overall performance when running benchmark tests suites execution timeno prefetcher IP C with prefetcher speedup f execution timewith prefetcher IPC prefetcher Good prefetch The prefetched block is referenced by the application be fore it is replaced Bad prefetch The prefetched block is replaced without being referenced Accuracy Accuracy measures the number of useful prefetches issued by the prefetcher goo
13. n detailed Detailed timing simulation caches Use caches 12cache Use level two cache 12size 1MB Level two cache size prefetcher policy proxy Use the C style prefetcher interface prefetcher on access True Have the cache notify the prefetcher on all accesses both hits and misses cmd The program an Alpha binary to run Table 2 1 Basic se py command line options build ALPHA SE m5 opt configs example se py detailed caches 12cache 12size 1MB prefetcher policy proxy prefetcher on access True This command will run se py with a default program which prints out Hello world and exits To run something more complicated use the cmd option to specify another program See subsection 2 4 2 about cross compiling binaries for the Alpha architecture Another possibility is to run a benchmark program as described in the next section 2 4 1 CPU2000 benchmark tests The test_prefetcher py script can be used to evaluate the performance of your prefetcher against the SPEC CPU2000 benchmarks It runs a selected suite of CPU2000 tests with your prefetcher and compares the results to some reference prefetchers The per test statistics that M5 generates are written to output lt testname prefetcher gt stats txt The statistics most relevant for hardware prefetching are then filtered and aggregated to a stats txt file in the framework base directory See chapter 4 for an explanation of the reported statistics
14. ontaining this address is then added to the prefetch request 12 queue This queue has a fixed limit of MAX QUEUE SIZE pending prefetch re quests Unless your prefetcher is using a high degree of prefetching the number of outstanding prefetches will stay well below this limit Every time the cache has loaded a block requested by the prefetcher prefetch complete is called with the address of the loaded block Other functionality available through the interface are the functions for get ting setting and clearing the prefetch bit Each cache block has one such tag bit You are free to use this bit as you see fit in your algorithms Note that this bit is not automatically set if the block has been prefetched it has to be set manually by calling set prefetch bit set prefetch bit on an address that is not in cache has no effect and get prefetch bit on an address that is not in cache will always return false When you are ready to write code for your prefetching algorithm of choice put it in prefetcher prefetcher cc When you have several prefetchers you may want to to make prefetcher cc a symlink The prefetcher is statically compiled into M5 After prefetcher cc has been changed recompile with compile sh No options needed 13 3 3 1 Example prefetcher sample prefetcher which does sequential one block lookahead This means that the prefetcher fetches the next block after the one that was just accessed It also i
15. rt Appliance in the file menu and opening Prefetcher framework ovf The root password of the virtual machine is user no quotation marks It will be required if you want to run sudo commands 2 3 Build M5 uses the scons build system scons j2 build ALPHA SE mb opt builds the optimized version of the M5 binaries j2 specifies that the build process should built two targets in parallel This is a useful option to cut down on compile time if your machine has several processors or cores The included build script compile sh encapsulates the necessary build com mands and options Since the g 4 version is rather old you can also compile the simulator on kongull Use module load scons amp amp module load swig to load the necessary programs check with module list whether they were loaded and then compile by invoking the build script 2 4 Run Before running M5 it is necessary to specify the architecture and parameters for the simulated system This is a nontrivial task in itself Fortunately there is an easy way use the included example python script for running M5 in syscall emulation mode m5 config example se py When using a prefetcher with M5 this script needs some extra options described in Table 2 1 For an overview of all possible options to se py do build ALPHA SE m5 opt configs example se py help When combining all these options the command line will look something like this Option Descriptio
16. ter queue Returns the number of queued prefetch requests Macro to print debug information trace is a trace flag HWPrefetch and format is a printf format string Table 3 3 Functions callable from the user defined prefetcher AccessStat member Description Addr pc The address of the instruction that caused the access Program Counter Addr mem_addr The memory address that was requested Tick time The simulator time cycle when the request was sent int miss Whether this demand access was a cache hit or miss Table 3 4 AccessStat members 11 The prefetcher must implement the three functions prefetch init prefetch access and prefetch complete The implementation may be empty The function prefetch init void is called at the start of the simulation to allow the prefetcher to initialize any data structures it will need When the L2 cache is accessed by the CPU through the L1 cache the func tion void prefetch access AccessStat stat is called with an argument AccessStat stat that gives various information about the access When the prefetcher decides to issue a prefetch request it should call issue prefetch Addr addr which queues up a prefetch request for the block containing addr When a cache block that was requested by issue prefetch arrives from memory prefetch complete is called with the address of the completed request as parameter Prefetches issued by issue prefetch Addr addr go into a prefetch
17. trace flags HWPrefetch Warning this can produce a lot of output It might be better to redirect stdout to file when running with trace flags enabled 5 2 GDB The GNU Project Debugger gdb can be used to inspect the state of the simulator while running and to investigate the cause of a crash Pass GDB the executable you want to debug when starting it gdb args m5 build ALPHA SE m5 debug remote gdb port 0 re outdir output ammp user m5 configs example se py checkpoint dir lib cp checkpoint restore 1000000000 at instruction caches 12cache standard switch warmup insts 10000000 max inst 10000000 12size 1MB bench ammp prefetcher on access true policy proxy You can then use the run command to start the executable 17 Some useful GDB commands run args Restart the executable with the given command line arguments run Restart the executable with the same arguments as last time where Show stack trace up Move up stack trace down Move down stack frame print lt expr gt Print the value of an expression help Get help for commands quit Exit GDB GDB has many other useful features for more information you can consult the GDB User Manual at http sourceware org gdb current onlinedocs gdb 5 3 Valgrind Valgrind is a very useful tool for memory debugging and memory leak detec tion If your prefetcher causes M5 to crash or behave strangely it is useful to run it under Valgrind and see if
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