= Data replication & distribution policy =

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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.  

==__1. general principles__==

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 without segmentation violation: if the corresponding page 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 an integer number of pages, as a given page cannot be shared by two different vsegs. 

In all UNIX system (including almos-mkh), a '''vseg''' has some specific attributes defining access rights (readable, writable, executable, catchable, etc). 
But for almos-mkh, the vseg type defines also the replication and 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'''.

The '''vseg''' structure and API is defined in the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/mm/vseg.h almos_mk/kernel/mm/vseg] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/mm/vseg.c almos-mkh/kernel/mm/vseg.c] files.

In all UNIX systems, the process descriptor contains the table used by the MMU to make the virtual to physical address translation. An important feature of almos-mkh is the following: To avoid contention, in parallel applications creating 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 this process. These clusters are called ''active'' clusters. 

In almos-mkh, the structure used by the MMU for address translation is called VMM (Virtual Memory Manager). 
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 virtual segments registered for process P in cluster K,
 * The '''GPT(P,K)''' is the generic page table, defining the actual physical mapping for each page of each vseg.

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'). 
 1. 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. 
 1. 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.  
  
We have the following properties for the '''private''' vsegs:
 * the VSL(P,K) contains always all private vsegs in cluster K, 
 * The GPT(P,K) contains all mapped entries corresponding to a private vseg in cluster K.

We have the following properties for the '''public''' vsegs:
 * the VSL(P,K) contains only the public vsegs that have been actually accessed by a thread of P running in cluster K.
 * Only the reference cluster KREF contains the complete VSL(P,KREF) of all public vsegs for the P process.
 * The GPT(P,K) contains only  the entries 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 the P process.

For the '''public''' vsegs,  the VMM(P,K) structures - other than the reference one - can be considered as local caches.
This creates a coherence problem, that is solved by the following rules :
1. 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.
1. When a given public vseg in the VSL, or a given entry in the GPT must be removed or modified, this modification must be done first in the reference cluster, and broadcast to all other clusters for update of local VSL or GPT copies.
1. 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 more details on the VMM implementation, the API is defined in the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/mm/vmm.h almos_mkh/kernel/mm/vmm.h] and [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/mm/vmm.c almos-mkh/kernel/mm/vmm.c] files.

== __2. User segments__  ==

This section describes the six types of user virtual segments and the associated replication / distribution policy defined and implemented by almost-mkh:

=== 2.1 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.

=== 2.2 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.

=== 2.3 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.

=== 2.4 ANON vseg ===

This '''public''' vseg is dynamically created by ALMOS-MK to serve an anonymous [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/syscalls/sys_mmap.c 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.

=== 2.5 FILE vseg ===

This '''public''' vseg is dynamically created by ALMOS-MK to serve a file based [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/syscalls/sys_mmap.c 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.

=== 2.6 REMOTE vseg ===

This '''public''' vseg is dynamically created by ALMOS-MK to serve a remote [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/syscalls/sys_mmap.c 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.

=== 2.7 summary ===

This table summarize the replication, distribution & mapping rules for user vsegs:

|| 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)  ||


== __ 3. 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.

=== 3.1. 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).

=== 3.2. 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.

=== 3.3. 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

=== 3.4. KDEV vsegs ===


== __4. 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  [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/hal/generic/hal_remote.h  almos_mkh/hal/generic/hal_remote.h] file.

The TSAR implementation is available in the  [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/hal/tsar_mips32/core/hal_remote.c  almos_mkh/hal/tsar_mips32/core.hal_remote.c] file.


=== 4.1 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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/hal/generic/hal_remote.h  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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/hal/tsar/mips32/core/hal_remote.c  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.


=== 4.2 Intel 64 bits ===

TODO

== __4. virtual space organisation__ ==

This section describes the almost-mkh assumptions regarding the virtual space organisation, that is strongly dependent on the size of the virtual space.  

=== 5.1 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.

'''5.1.1 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.
 
'''5.1.2 The ''utils'' zone'''

It contains the two ''args'' and ''envs'' vsegs, whose sizes are defined by [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/kernel_config. 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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/kernel_config.h  CONFIG_VMM_ELF_BASE] parameter.

'''5.1.3 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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/tools/arch_info/boot_info.h boot_info_t] structure by the boot loader.

'''5.1.4 The ''heap'' zone'''

It contains all vsegs dynamically allocated / released  by the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/syscalls/sys_mmap.c mmap] / [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/syscalls/sys_munmap.c 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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/kernel_config.h 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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/libs/libalmosmkh/almosmkh.c 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.  

'''5.1.5 The ''stack'' zone'''

It is located on top of the '''mmap''' zone and starts at the address defined by the [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/kernel_config.h 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 [https://www-soc.lip6.fr/trac/almos-mkh/browser/trunk/kernel/kernel_config.h 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