diff options
Diffstat (limited to 'Documentation/x86/intel_rdt_ui.txt')
| -rw-r--r-- | Documentation/x86/intel_rdt_ui.txt | 75 |
1 files changed, 67 insertions, 8 deletions
diff --git a/Documentation/x86/intel_rdt_ui.txt b/Documentation/x86/intel_rdt_ui.txt index 71c30984e94d5..a16aa21138402 100644 --- a/Documentation/x86/intel_rdt_ui.txt +++ b/Documentation/x86/intel_rdt_ui.txt @@ -17,12 +17,14 @@ MBA (Memory Bandwidth Allocation) - "mba" To use the feature mount the file system: - # mount -t resctrl resctrl [-o cdp[,cdpl2]] /sys/fs/resctrl + # mount -t resctrl resctrl [-o cdp[,cdpl2][,mba_MBps]] /sys/fs/resctrl mount options are: "cdp": Enable code/data prioritization in L3 cache allocations. "cdpl2": Enable code/data prioritization in L2 cache allocations. +"mba_MBps": Enable the MBA Software Controller(mba_sc) to specify MBA + bandwidth in MBps L2 and L3 CDP are controlled seperately. @@ -270,10 +272,11 @@ and 0xA are not. On a system with a 20-bit mask each bit represents 5% of the capacity of the cache. You could partition the cache into four equal parts with masks: 0x1f, 0x3e0, 0x7c00, 0xf8000. -Memory bandwidth(b/w) percentage --------------------------------- -For Memory b/w resource, user controls the resource by indicating the -percentage of total memory b/w. +Memory bandwidth Allocation and monitoring +------------------------------------------ + +For Memory bandwidth resource, by default the user controls the resource +by indicating the percentage of total memory bandwidth. The minimum bandwidth percentage value for each cpu model is predefined and can be looked up through "info/MB/min_bandwidth". The bandwidth @@ -285,7 +288,47 @@ to the next control step available on the hardware. The bandwidth throttling is a core specific mechanism on some of Intel SKUs. Using a high bandwidth and a low bandwidth setting on two threads sharing a core will result in both threads being throttled to use the -low bandwidth. +low bandwidth. The fact that Memory bandwidth allocation(MBA) is a core +specific mechanism where as memory bandwidth monitoring(MBM) is done at +the package level may lead to confusion when users try to apply control +via the MBA and then monitor the bandwidth to see if the controls are +effective. Below are such scenarios: + +1. User may *not* see increase in actual bandwidth when percentage + values are increased: + +This can occur when aggregate L2 external bandwidth is more than L3 +external bandwidth. Consider an SKL SKU with 24 cores on a package and +where L2 external is 10GBps (hence aggregate L2 external bandwidth is +240GBps) and L3 external bandwidth is 100GBps. Now a workload with '20 +threads, having 50% bandwidth, each consuming 5GBps' consumes the max L3 +bandwidth of 100GBps although the percentage value specified is only 50% +<< 100%. Hence increasing the bandwidth percentage will not yeild any +more bandwidth. This is because although the L2 external bandwidth still +has capacity, the L3 external bandwidth is fully used. Also note that +this would be dependent on number of cores the benchmark is run on. + +2. Same bandwidth percentage may mean different actual bandwidth + depending on # of threads: + +For the same SKU in #1, a 'single thread, with 10% bandwidth' and '4 +thread, with 10% bandwidth' can consume upto 10GBps and 40GBps although +they have same percentage bandwidth of 10%. This is simply because as +threads start using more cores in an rdtgroup, the actual bandwidth may +increase or vary although user specified bandwidth percentage is same. + +In order to mitigate this and make the interface more user friendly, +resctrl added support for specifying the bandwidth in MBps as well. The +kernel underneath would use a software feedback mechanism or a "Software +Controller(mba_sc)" which reads the actual bandwidth using MBM counters +and adjust the memowy bandwidth percentages to ensure + + "actual bandwidth < user specified bandwidth". + +By default, the schemata would take the bandwidth percentage values +where as user can switch to the "MBA software controller" mode using +a mount option 'mba_MBps'. The schemata format is specified in the below +sections. L3 schemata file details (code and data prioritization disabled) ---------------------------------------------------------------- @@ -308,13 +351,20 @@ schemata format is always: L2:<cache_id0>=<cbm>;<cache_id1>=<cbm>;... -Memory b/w Allocation details ------------------------------ +Memory bandwidth Allocation (default mode) +------------------------------------------ Memory b/w domain is L3 cache. MB:<cache_id0>=bandwidth0;<cache_id1>=bandwidth1;... +Memory bandwidth Allocation specified in MBps +--------------------------------------------- + +Memory bandwidth domain is L3 cache. + + MB:<cache_id0>=bw_MBps0;<cache_id1>=bw_MBps1;... + Reading/writing the schemata file --------------------------------- Reading the schemata file will show the state of all resources @@ -358,6 +408,15 @@ allocations can overlap or not. The allocations specifies the maximum b/w that the group may be able to use and the system admin can configure the b/w accordingly. +If the MBA is specified in MB(megabytes) then user can enter the max b/w in MB +rather than the percentage values. + +# echo "L3:0=3;1=c\nMB:0=1024;1=500" > /sys/fs/resctrl/p0/schemata +# echo "L3:0=3;1=3\nMB:0=1024;1=500" > /sys/fs/resctrl/p1/schemata + +In the above example the tasks in "p1" and "p0" on socket 0 would use a max b/w +of 1024MB where as on socket 1 they would use 500MB. + Example 2 --------- Again two sockets, but this time with a more realistic 20-bit mask. |