Data & Instruction Address Restriction
From PAPIDocs
Jump to: navigation, search

DATA AND INSTRUCTION ADDRESS RESTRICTION

Introduction

Performance instrumentation of data structures, as opposed to code segments, is a feature not widely supported across a range of platforms. One platform on which this feature is supported is the Itanium2. In fact, event counting on Itanium2 can be qualified by a number of conditioners, including instruction address, opcode matching, and data address. We have implemented a generalized PAPI interface for data structure and instruction range performance instrumentation, also referred to as data and instruction range specification, and applied that interface to the specific instance of the Itanium2 platform to demonstrate its viability. This feature is being introduced for the first time in the PAPI 3.5 release.


The PAPI Interface

Since PAPI is a platform-independent library, care must be taken when extending its feature set so as not to disrupt the existing interface or to clutter the API with calls to functionality that is not available on a large subset of the supported platforms. To that end, we elected to extend an existing PAPI call, PAPI_set_opt(), with the capability of specifying starting and ending addresses of data structures or instructions to be instrumented. The PAPI_set_opt() call previously supported functionality to set a variety of optional capability in the PAPI interface, including debug levels, multiplexing of eventsets, and the scope of counting domains. This call was extended with two new cases to support instruction and data address range specification: PAPI_INSTR_ADDRESS and PAPI_DATA_ADDRESS. To access these options, a user initializes a simple option specific data structure and calls PAPI_set_opt() as illustrated in the code fragment below:

   ...
   option.addr.eventset = EventSet;
   option.addr.start = (caddr_t)array;
   option.addr.end = (caddr_t)(array + size_array);
   retval = PAPI_set_opt(PAPI_DATA_ADDRESS, &option);
   ...

The user creates a PAPI eventset and determines the starting and ending addresses of the data to be monitored. The call to PAPI_set_opt then prepares the interface to count events that occur on accesses to data in that range. The specific events to be monitored can be added to the eventset either before or after the data range is specified. In a similar fashion, an instruction range can be set using the PAPI_INSTR_ADDRESS option. If this option is supported on the platform in use, the data is transferred to the platform-specific implementation and handled appropriately. If not supported, the call returns an error message.

It may not always be possible to exactly specify the address range of interest. If this is the case, it is important that the user have some way to know what approximations have been made, so that appropriate corrective action can be taken. For instance, to isolate a specific data structure completely, it may be necessary to pad memory before and after the structure with dummy structures that are never accessed. To facilitate this, PAPI_set_opt() returns the offsets from the requested starting and ending addresses as they were actually programmed into the hardware. If the addresses were mapped exactly, these values are zero. An example of this is shown below:

   ...
   retval = PAPI_set_opt(PAPI_DATA_ADDRESS, &option);
   actual.start = (caddr_t)array - option.addr.start_off;
   actual.end = (caddr_t)(array + size_array) + option.addr.end_off;
   ...

Itanium Idiosyncrasies

There are roughly 475 native events available on Itanium 2.160 of them are memory related and can be counted with data address specification in place. 283 can be counted using instruction address specification. All events in an eventset with data or instruction range specification in place must be one of these supported events. Further restrictions also apply to the use of data and instruction range specification, as described below. Data addresses can only be specified in coarse mode. Although four independent pairs of data address registers exist in the hardware and would suggest that four disjoint address regions can be monitored simultaneously, the Intel documentation strongly suggests that this is not a good idea. Further, the underlying software takes advantage of these register pairs to tune the range of addresses that is actually monitored. See the discussion under Data Address Ranges for further detail.


Instruction Address Ranges

Instruction ranges can be specified in one of two ways: coarse and fine. In fine mode, addresses can be specified exactly, but both the start and end addresses must exist on the same 4K byte page. In other words, the address range must be less than 4K bytes, and the addresses can only differ in the bottom 12 bits. If fine mode cannot be used, the underlying perfmon library automatically switches to coarse address specification. Four pairs of registers are available to specify coarse instruction address ranges. The restrictions to coarse address specification are discussed below.


Data Address Ranges

Data addresses can only be specified in coarse mode. As with instruction ranges, four pairs of registers are available to specify the data address ranges. Use of coarse mode addressing for either instruction or data address specification can cause some anomalous results. The Intel documentation points out that starting and ending addresses cannot be specified exactly, since the hardware representation relies on powers-of-two bitmasks. The perfmon library tries to optimize the alignment of these power-of-two regions to cover the addresses requested as effectively as possible with the four sets of registers available. Perfmon first finds the largest power-of-two address region completely contained within the requested addresses. Then it finds successively smaller power-of-two regions to cover the errors on the high and low end of the requested address range. The effective result is that the actual range specified is always equal to or larger than, and completely contains the requested range, and can occupy from one to four pairs of address registers. In some cases this can result in significant overcounts of the events of interest, especially if two active data structures are located in close proximity to each other. This may require that the developer insert some padding structures before and/or after a particular structure of interest to guarantee accurate counts.


Supporting Software

To make this new PAPI feature more accessible and easier to use, a test case was developed to both provide a coding example and to exercise and test the fuctionality of the data ranging features of the Itanium 2. In addition, the papi_native_event utility was modified to make it easier to identify events that support these features.


The data_range.c Test Case

A test case, called data_range was developed that measures memory load and store events on three different types of data structures. Three static arrays of 16,384 ints were declared in the program, and three dynamic arrays of 16,384 ints were malloc'd. The data range was specified sequentially to be the starting and ending addresses of each of:

the pointers to the malloc'd arrays;

the malloc'd arrays themselves;

the statically declared arrays.

The work done in each case consisted of loading an initialization value into each element of each array, and then summing the values of each element. This should produce 16,384 loads and 16,384 stores on each element.

For the pointers, the size was 8 bytes and the starting and ending addresses could be specified exactly. Output is shown below:

Measure loads and stores on the pointers to the allocated arrays Expected loads: 32768; Expected stores: 0 These loads result from accessing the pointers to compute array addresses. They will likely disappear with higher levels of optimization. Requested Start Address: 0x6000000000011640; Start Offset: 0x 0; Actual Start Address: 0x6000000000011640 Requested End Address: 0x6000000000011648; End Offset: 0x 0; Actual End Address: 0x6000000000011648 loads_retired: 32768 stores_retired: 0 Requested Start Address: 0x6000000000011628; Start Offset: 0x 0; Actual Start Address: 0x6000000000011628 Requested End Address: 0x6000000000011630; End Offset: 0x 0; Actual End Address: 0x6000000000011630 loads_retired: 32768 stores_retired: 0 Requested Start Address: 0x6000000000011638; Start Offset: 0x 0; Actual Start Address: 0x6000000000011638 Requested End Address: 0x6000000000011640; End Offset: 0x 0; Actual End Address: 0x6000000000011640 loads_retired: 32768 stores_retired: 0

For the allocated arrays, small offsets were introduced in each case, and the resulting error in the loads and stores is exactly what would be predicted by the activity in the adjacent memory locations:

Measure loads and stores on the allocated arrays themselves Expected loads: 16384; Expected stores: 16384 Requested Start Address: 0x6000000004044010; Start Offset: 0x 10; Actual Start Address: 0x6000000004044000 Requested End Address: 0x6000000004054010; End Offset: 0x 0; Actual End Address: 0x6000000004054010 loads_retired: 16384 stores_retired: 16384 Requested Start Address: 0x6000000004054020; Start Offset: 0x 20; Actual Start Address: 0x6000000004054000 Requested End Address: 0x6000000004064020; End Offset: 0x 0; Actual End Address: 0x6000000004064020 loads_retired: 16388 stores_retired: 16388 Requested Start Address: 0x6000000004064030; Start Offset: 0x 30; Actual Start Address: 0x6000000004064000 Requested End Address: 0x6000000004074030; End Offset: 0x 10; Actual End Address: 0x6000000004074040 loads_retired: 16392 stores_retired: 16392

For the static arrays, the locations of the arrays resulted in significant offsets, and hence significant errors. The most interesting case is the second one, in which the starting offset can be seen to force the inclusion of all three pointers to the malloc'd arrays. Because of this the loads retired count is too high by 98310, almost exactly 3 * 32768 = 98204:

Measure loads and stores on the static arrays These values will differ from the expected values by the size of the offsets. Expected loads: 16384; Expected stores: 16384 Requested Start Address: 0x60000000000218cc; Start Offset: 0x 18cc; Actual Start Address: 0x6000000000020000 Requested End Address: 0x60000000000318cc; End Offset: 0x 734; Actual End Address: 0x6000000000032000 loads_retired: 18432 stores_retired: 18432 Requested Start Address: 0x60000000000118cc; Start Offset: 0x 18cc; Actual Start Address: 0x6000000000010000 Requested End Address: 0x60000000000218cc; End Offset: 0x 734; Actual End Address: 0x6000000000022000 loads_retired: 115155 stores_retired: 16845 Requested Start Address: 0x60000000000318cc; Start Offset: 0x 18cc; Actual Start Address: 0x6000000000030000 Requested End Address: 0x60000000000418cc; End Offset: 0x 734; Actual End Address: 0x6000000000042000 loads_retired: 17971 stores_retired: 17971


The papi_native_avail Utility

To effectively use the instruction and address range specification feature for Itanium 2, one must know which of the roughly 475 available native events support these features. In addition, there are other qualifiers to Itanium 2 native events that are valuable to inspect. For these reasons, the papi_native_avail utility was enhanced to make it possible to filter the list of native events by these qualifiers. A help feature was added to this utility to make it easier to remember the Itanium specific options:

> papi_native_avail --help This is the PAPI native avail program. It provides availability and detail information for PAPI native events. Usage: papi_native_avail [options] Options: -h, --help print this help message --darr display Itanium events that support Data Address Range Restriction --dear display Itanium Data Event Address Register events only --iarr display Itanium events that support Instruction Address Range Restriction --iear display Itanium Instruction Event Address Register events only --opcm display Itanium events that support OpCode Matching NOTE: The last five options are mutually exclusive.

If any of these options are specified on the command line, only those events that support that option are displayed. Even so, the list can be extensive, with roughly 160 events supporting data address ranging, and even more supporting instruction address ranging.