wiki:replication_distribution

Version 52 (modified by alain, 4 years ago) (diff)

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Data replication & distribution policy

  1. Data replication & distribution policy
    1. 1. User segments
      1. 1.1 CODE vsegs
      2. 1.2 DATA vseg
      3. 1.3 STACK vseg
      4. 1.4 ANON vseg
      5. 1.5 FILE vseg
      6. 1.6 REMOTE vseg
    2. 2. kernel segments
      1. 2.1. KCODE vsegs
      2. 2.2. KDATA vsegs
      3. 2.3. KHEAP vsegs
      4. 2.4. KDEV vsegs
      5. 2.1 TSAR-MIPS32
      6. 3.2 Intel 64 bits
    3. 4. virtual space organisation
      1. 4.1 TSAR-MIP32
      2. 4.2 Intel 64 bits

The replication / distribution policy of data on the physical memory banks has two goals: enforce locality (as much as possible), and avoid contention (it is the main goal).

The data to be placed are the virtual segments defined - at compilation time - in the virtual space of the various user processes currently running, or in the virtual space of the operating system itself.

To actually control the placement of all these virtual segments on the physical memory banks, the kernel uses the paged virtual memory MMU to map a virtual segment to a given physical memory bank in a given cluster.

A vseg is a contiguous memory zone in the process virtual space, defined by the two (base, size) values. All adresses in this interval can be accessed by in this process without segmentation violation: if the corresponding is not mapped, the page fault will be handled by the kernel, and a physical page will be dynamically allocated (and initialized if required). A vseg always occupies always an integer number of pages, as a given page cannot be shared by two different vsegs.

Depending on its type, a vseg has some specific attributes regarding access rights, and defining the replication and/or distribution policy:

  • A vseg is public when it can be accessed by any thread T of the involved process, whatever the cluster running the thread T. It is private when it can only be accessed by the threads running in the cluster containing the physical memory bank where this vseg is defined and mapped.
  • For a public vseg, ALMOS-MKH implements a global mapping : In all clusters, a given virtual address is mapped to the same physical address. For a private vseg, ALMOS-MKH implements a local mapping : the same virtual address can be mapped to different physical addresses, in different clusters.
  • A public vseg can be localized (all vseg pages are mapped in the same cluster), or distributed (different pages are mapped on different clusters). A private vseg is always localized.

To avoid contention, in case of parallel applications defining a large number of threads in one single process P, almos-mkh replicates, the process descriptor in all clusters containing at least one thread of P, and these clusters are called active clusters. Each process descriptor contains a VMM structure (Virtual Memory Manager) that register all informations required by the MMU to make the address translation for the process. For a process P in cluster K, the VMM(P,K) structure, contains two main sub-structures:

  • The ​VSL(P,K) is the list of all vsegs registered for process P in cluster K,
  • The ​GPT(P,K) is the generic page table, defining the actual physical mapping of these vsegs.

For a given process P, the different VMM(P,K) in different clusters can have different contents for several reasons :

  1. A private vseg can be registered in only one VSL(P,K) in cluster K, and be totally undefined in the others VSL(P,K').
  2. A public vseg can be replicated in deveral VSL(P,K), but the registration of a vseg in a given VSL(P,K) is on demand: the vseg is only registered in VSL(P,K) when a thread of process P running in cluster K try to access this vseg.
  3. Similarly, the mapping of a given virtual page VPN of a given vseg (i.e. the allocation of a physical page PPN to a virtual page VPN, and the registration of this PPN in the GPT(P,K) is on demand: the page table entry will be updated in the GPT(P,K) only when a thread of process P in cluster K try to access this VPN.

The replication of the VSL(P,K) and GPT(P,K) kernel structures in several clusters creates a coherence problem for the public vsegs:

  • A VSL(P,K) contains all private vsegs in cluster K, but contains only the public vsegs that have been actually accessed by a thread of P running in cluster K. Only the reference process descriptor stored in the reference cluster KREF contains the complete list VSL(P,KREF) of all public vsegs for the P process.
  • A GPT(P,K) contains all mapped entries corresponding to private vsegs but for public vsegs, it contains only the entries corresponding to pages that have been accessed by a thread running in cluster K. Only the reference cluster KREF contains the complete GPT(P,KREF) of all mapped entries of public vsegs for process P.

Therefore, almos-mkh defines the following rules :

For the public vsegs, the VMM(P,K) structures - other than the reference one - can be considered as read-only caches. When a given vseg or a given entry in the page table must be removed by the kernel, this modification must be done first in the reference cluster, and broadcast to all other clusters for update. When a miss is detected in a non-reference cluster, the reference VMM(P,KREF) must be accessed first to check a possible false segmentation fault or a false page fault.

For the private vsegs, and the corresponding entries in the page table, the VSL(P,K) and the GPT(P,K) are only shared by the threads of P running in cluster K, and these structures can be privately handled by the local kernel instance in cluster K.

For more details on implementation:

The vseg API is defined in the ​almos_mk/kernel/mm/vseg and ​almos-mkh/kernel/mm/vseg.c files.

The Virtual Memory Manager API is defined in the ​almos_mkh/kernel/mm/vmm.h and ​almos-mkh/kernel/mm/vmm.c files.

1. User segments

This section describes the six types of user virtual segments defined by almost-mkh:

1.1 CODE vsegs

This private vseg contains the application code. It is replicated in all clusters. ALMOS-MK creates one CODE vseg per active cluster. For a process P, the CODE vseg is registered in the VSL(P,KREF) when the process is created in reference cluster KREF. In the other clusters K, the CODE vseg is registered in VSL(P,K) when a page fault is signaled by a thread of P running in cluster K. In each active cluster K, the CODE vseg is mapped in cluster K.

1.2 DATA vseg

This public vseg contains the user application global data. ALMOS-MK creates one single DATA vseg, that is registered in the reference VSL(P,KREF) when the process P is created in reference cluster KREF. In the other clusters K, the DATA vseg is registered in VSL(P,K) when a page fault is signaled by a thread of P running in cluster K. To avoid contention, this vseg is physically distributed on all clusters, with a page granularity. For each page, the physical mapping is defined by the LSB bits of the VPN.

1.3 STACK vseg

This private vseg contains the execution stack of a thread. Almos-mkh creates one STACK vseg for each thread of P running in cluster K. This vseg is registered in the VSL(P,K) when the thread descriptor is created in cluster K. To enforce locality, this vseg is of course mapped in cluster K.

1.4 ANON vseg

This public vseg is dynamically created by ALMOS-MK to serve an anonymous ​mmap system call executed by a client thread running in a cluster K. The vseg is registered in VSL(P,KREF), but the vseg is mapped in the client cluster K.

1.5 FILE vseg

This public vseg is dynamically created by ALMOS-MK to serve a file based ​mmap system call executed by a client thread running in a cluster K. The vseg is registered in VSL(P,KREF), but the vseg is mapped in cluster Y containing the file cache.

1.6 REMOTE vseg

This public vseg is dynamically created by ALMOS-MK to serve a remote ​mmap system call, where a client thread running in a cluster X requests to create a new vseg mapped in another cluster Y. The vseg is registered in VSL(P,KREF), but the vseg is mapped in cluster Y specified by the user.

This table summarize the user vsegs features:

Type Access Replication Mapping in physical space Allocation policy in virtual space
STACK private localized Read Write one per thread same cluster as thread using it dynamic (one stack allocator per cluster)
CODE private localized Read Only one per cluste same cluster as thread using it static (defined in .elf file)
DATA public distributed Read Write non replicated distributed on all clusters static (defined in .elf file)
ANON public localized Read Write non replicated same cluster as calling thread dynamic (one heap allocator per process
FILE public localized Read Write non replicated same cluster as the file cache dynamic (one heap allocator per process)
REMOTE public localized Read Write non replicated cluster defined by user dynamic (one heap allocator per process)

2. kernel segments

For any process descriptor P in a cluster K, the VMM(P,K) contains not only the user vsegs defined above, but also the kernel vsegs, because all user theads can make system calls, that must access both the kernel instructions and the kernel data structures, and this requires address translation. This section describes the four types of kernel virtual segments defined by almost-mkh.

2.1. KCODE vsegs

The KCODE vseg contains the kernel code defined in the kernel.elf file. Almos-mkh creates one KCODE vseg in each cluster K, to avoid contention. It is a private vseg, that is accessed only by the threads running in cluster K. It can be an user thread executing a syscall, or it can be a specialized kernel thread (such as an IDLE thread, a DEV thread, or a RPC thread). In each cluster K, the KCODE vseg is registered in the VMM(0,K) associated to the kernel process_zero, that contains all kernel threads, and in the VMM(P,K) of each user process P that has at least one thread running in cluster K. This vseg uses only big pages, that are mapped by the kernel_init function (no on demand paging for this vseg).

2.2. KDATA vsegs

This public vseg contains the global data, statically allocated at compilation time. This vseg is also replicated in all clusters. The values initially contained in these KDATA vsegs are identical, as they are defined in the kernel.elf file. But they are not read-only, and can evolve differently in different clusters. As the KDATA vsegs are replicated in all clusters, most accesses to a KDATA segment are expected to be done by local threads. These local accesses can use the normal pointers in virtual kernel space.

But there is only one vseg defined in the kernel.elf file, and there is as many KDATA segments as the number of clusters. Even if most accesses are local, a thread running in cluster K must be able to access a global variable stored in another cluster X, or to send a request to another kernel instance in cluster X, or to scan a globally distributed structure, such as the DQDT or the VFS. To support this cooperation between kernel instances, almos-mkh defines the remote_load( cxy , ptr ) and remote_store( cxy , ptr ) functions, where ptr is a normal pointer in kernel virtual space on a variable stored in the KDATA vseg, and cxy is the remote cluster identifier. Notice that a given global variable is now identified by and extended pointer XPTR( cry , ptr ). With these remote access primitives, any kernel instance in cluster K can access any global variable in any cluster.

2.3. KHEAP vsegs

Beside the statically allocated global variables, a large number of kernel structures, such as the user process descriptors, the thread descriptors, the vseg descriptors, the file descriptors, etc. are dynamically descriptors

2.4. KDEV vsegs

==3. Physical mapping of kernel vsegs ==

The implementation of these remote access functions depends on the target architecture.

rewant to communicate withA the writable and readable bi any functionin this vseg are identical in all clusters, and defined bbut this vseg being read/write,

the contentglobal data. ALMOS-MK creates one DATA vseg in each vseg, that is registered in the reference VSL(P,KREF) when the process P is created in reference cluster KREF. In the other clusters K, the DATA vseg is registered in VSL(P,K) when a page fault is signaled by a thread of P running in cluster K. To avoid contention, this vseg is physically distributed on all clusters, with a page granularity. For each page, the physical mapping is defined by the LSB bits of the page VPN.* The read-only segment containing the user code is replicated in all clusters where there is at least one thread using it.

The remote_access primitives are defined in the ​almos_mkh/hal/generic/hal_remote.h file.

The TSAR implementation is available in the ​almos_mkh/hal/tsar_mips32/core.hal_remote.c file.

2.1 TSAR-MIPS32

As the TSAR architecture uses 32 bits cores, to reduce the power consumption, the virtual space is much smaller (4 Gbytes) than the physical space. Even if

To enforce locality, there is one KDATA segment per cluster, containing a copy of all global variables statically allocated at compilation time. On the other hand, all structures dynamically allocated by the kernel (to create a new process descriptor, a new thread descriptor, a new file descriptor, etc.) are allocated in the KHEAP segment of the target cluster, and will be mainly handled by a kernel instance running in this same kernel. Therefore, most kernel memory accesses expected to be local.

In the - rare - situations where the kernel running in cluster K must access data in a remote cluster K' (to access a globally distributed structure such as the DQDT, or for inter-cluster client/server communication) almos-mkh uses specific remote access primitives defined in the ​hal_remote.h file.

In the TSAR architecture, and for any process P in any cluster K, almost-mkh registers only one extra KCODE vseg in the VMM[P,K), because almos-mkh does not use the DATA-MMU during kernel execution : Each time a core enters the kernel, to handle a sys call, an interrupt, or an exception, the DATA-MMU is deactivated, and It is reactivated when the core returns to user code.

The architecture dependent ​remote access functions use the TSAR specific extension register to build a 40 bits physical address from the 32 bits virtual address.

This pseudo identity mapping impose some constraints on the KCODE vseg.

3.2 Intel 64 bits

TODO

4. virtual space organisation

4.1 TSAR-MIP32

The virtual address space of an user process P is split in 5 fixed size zones, defined by configuration parameters in ​https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/kernel_config.h. Each zone contains one or several vsegs, as described below.

4.1.1 The kernel zone

It contains the kcode vseg (type KCODE), that must be mapped in all user processes. It is located in the lower part of the virtual space, and starts a address 0. Its size cannot be less than a big page size (2 Mbytes for the TSAR architecture), because it will be mapped as one (or several big) pages. 4.1.2 The utils zone

It contains the two args and envs vsegs, whose sizes are defined by ​specific configuration parameters. The args vseg (DATA type) contains the process main() arguments. The envs vseg (DATA type) contains the process environment variables. It is located on top of the kernel zone, and starts at address defined by the ​CONFIG_VMM_ELF_BASE parameter.

4.1.3 The elf zone

It contains the text (CODE type) and data (DATA type) vsegs, defining the process binary code and global data. The actual vsegs base addresses and sizes are defined in the .elf file and reported in the ​boot_info_t structure by the boot loader.

4.1.4 The heap zone

It contains all vsegs dynamically allocated / released by the ​mmap / ​munmap system calls (i.e. FILE / ANON / REMOTE types). It is located on top of the elf zone, and starts at the address defined by the ​CONFIG_VMM_HEAP_BASE parameter. The VMM defines a specific MMAP allocator for this zone, implementing the buddy algorithm. The mmap( FILE ) syscall maps directly a file in user space. The user level malloc library uses the mmap( ANON ) syscall to allocate virtual memory from the heap and map it in the same cluster as the calling thread. Besides the standard malloc() function, this library implements a non-standard ​remote_malloc() function, that uses the mmap( REMOTE ) syscall to dynamically allocate virtual memory from the heap, and map it to a remote physical cluster.

4.1.5 The stack zone

It is located on top of the mmap zone and starts at the address defined by the ​CONFIG_VMM_STACK_BASE parameter. It contains an array of fixed size slots, and each slot contains one stack vseg. The size of a slot is defined by the ​CONFIG_VMM_STACK_SIZE. In each slot, the first page is not mapped, in order to detect stack overflows. As threads are dynamically created and destroyed, the VMM implements a specific STACK allocator for this zone, using a bitmap vector. As the stack vsegs are private (the same virtual address can have different mappings, depending on the cluster) the number of slots in the stack zone actually defines the max number of threads for given process in a given cluster.

4.2 Intel 64 bits

TODO