There Are Not Enough Slots Available In The System To Satisfy

by admin

版本: Open MPI 3.0.1. 编译好可执行的C语言程序后，使用 mpirun -np 3 Test 命令，发现没有正常运行，而是报错：. There are not enough slots available in the system to satisfy the 3 slots. There are not enough slots available in the system to satisfy the 8 slots Either request fewer slots for your application, or make more slots available for use.

  1. There Are Not Enough Slots Available In The System To Satisfy New
  2. Horovod There Are Not Enough Slots Available In The System To Satisfy
  3. There Are Not Enough Slots Available In The System To Satisfy Free

Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous, so it cannot be used. 2: Internal fragmentation. Memory block assigned to process is bigger. Some portion of memory is left unused, as it cannot be used by another process.

Google is committed to advancing racial equity for Black communities. See how.

A/B system updates, also known as seamless updates, ensure a workable booting system remains on the disk during an over-the-air (OTA) update. This approach reduces the likelihood of an inactive device after an update, which means fewer device replacements and device reflashes at repair and warranty centers. Other commercial-grade operating systems such as ChromeOS also use A/B updates successfully.

For more information about A/B system updates and how they work, see Partition selection (slots).

A/B system updates provide the following benefits:

  • OTA updates can occur while the system is running, without interrupting the user. Users can continue to use their devices during an OTA—the only downtime during an update is when the device reboots into the updated disk partition.
  • After an update, rebooting takes no longer than a regular reboot.
  • If an OTA fails to apply (for example, because of a bad flash), the user will not be affected. The user will continue to run the old OS, and the client is free to re-attempt the update.
  • If an OTA update is applied but fails to boot, the device will reboot back into the old partition and remains usable. The client is free to re-attempt the update.
  • Any errors (such as I/O errors) affect only the unused partition set and can be retried. Such errors also become less likely because the I/O load is deliberately low to avoid degrading the user experience.
  • Updates can be streamed to A/B devices, removing the need to download the package before installing it. Streaming means it's not necessary for the user to have enough free space to store the update package on /data or /cache.
  • The cache partition is no longer used to store OTA update packages, so there is no need to ensure that the cache partition is large enough for future updates.
  • dm-verity guarantees a device will boot an uncorrupted image. If a device doesn't boot due to a bad OTA or dm-verity issue, the device can reboot into an old image. (Android Verified Boot does not require A/B updates.)

About A/B system updates

A/B updates require changes to both the client and the system. The OTA package server, however, should not require changes: update packages are still served over HTTPS. For devices using Google's OTA infrastructure, the system changes are all in AOSP, and the client code is provided by Google Play services. OEMs not using Google's OTA infrastructure will be able to reuse the AOSP system code but will need to supply their own client.

For OEMs supplying their own client, the client needs to:

  • Decide when to take an update. Because A/B updates happen in the background, they are no longer user-initiated. To avoid disrupting users, it is recommended that updates are scheduled when the device is in idle maintenance mode, such as overnight, and on Wi-Fi. However, your client can use any heuristics you want.
  • Check in with your OTA package servers and determine whether an update is available. This should be mostly the same as your existing client code, except that you will want to signal that the device supports A/B. (Google's client also includes a Check now button for users to check for the latest update.)
  • Call update_engine with the HTTPS URL for your update package, assuming one is available. update_engine will update the raw blocks on the currently unused partition as it streams the update package.
  • Report installation successes or failures to your servers, based on the update_engine result code. If the update is applied successfully, update_engine will tell the bootloader to boot into the new OS on the next reboot. The bootloader will fallback to the old OS if the new OS fails to boot, so no work is required from the client. If the update fails, the client needs to decide when (and whether) to try again, based on the detailed error code. For example, a good client could recognize that a partial ('diff') OTA package fails and try a full OTA package instead.

Optionally, the client can:

  • Show a notification asking the user to reboot. If you want to implement a policy where the user is encouraged to routinely update, then this notification can be added to your client. If the client does not prompt users, then users will get the update next time they reboot anyway. (Google's client has a per-update configurable delay.)
  • Show a notification telling users whether they booted into a new OS version or whether they were expected to do so but fell back to the old OS version. (Google's client typically does neither.)

On the system side, A/B system updates affect the following:

  • Partition selection (slots), the update_engine daemon, and bootloader interactions (described below)
  • Build process and OTA update package generation (described in Implementing A/B Updates)
Note: A/B system updates implemented through OTA are recommended for new devices only.

Partition selection (slots)

A/B system updates use two sets of partitions referred to as slots (normally slot A and slot B). The system runs from the current slot while the partitions in the unused slot are not accessed by the running system during normal operation. This approach makes updates fault resistant by keeping the unused slot as a fallback: If an error occurs during or immediately after an update, the system can rollback to the old slot and continue to have a working system. To achieve this goal, no partition used by the current slot should be updated as part of the OTA update (including partitions for which there is only one copy).

Each slot has a bootable attribute that states whether the slot contains a correct system from which the device can boot. The current slot is bootable when the system is running, but the other slot may have an old (still correct) version of the system, a newer version, or invalid data. Regardless of what the current slot is, there is one slot that is the active slot (the one the bootloader will boot form on the next boot) or the preferred slot.

Each slot also has a successful attribute set by the user space, which is relevant only if the slot is also bootable. A successful slot should be able to boot, run, and update itself. A bootable slot that was not marked as successful (after several attempts were made to boot from it) should be marked as unbootable by the bootloader, including changing the active slot to another bootable slot (normally to the slot running immediately before the attempt to boot into the new, active one). The specific details of the interface are defined in boot_control.h.

Update engine daemon

A/B system updates use a background daemon called update_engine to prepare the system to boot into a new, updated version. This daemon can perform the following actions:

  • Read from the current slot A/B partitions and write any data to the unused slot A/B partitions as instructed by the OTA package.
  • Call the boot_control interface in a pre-defined workflow.
  • Run a post-install program from the new partition after writing all the unused slot partitions, as instructed by the OTA package. (For details, see Post-installation).

As the update_engine daemon is not involved in the boot process itself, it is limited in what it can do during an update by the SELinux policies and features in the current slot (such policies and features can't be updated until the system boots into a new version). To maintain a robust system, the update process should not modify the partition table, the contents of partitions in the current slot, or the contents of non-A/B partitions that can't be wiped with a factory reset.

Update engine source

The update_engine source is located in system/update_engine. The A/B OTA dexopt files are split between installd and a package manager:

  • frameworks/native/cmds/installd/ota* includes the postinstall script, the binary for chroot, the installd clone that calls dex2oat, the post-OTA move-artifacts script, and the rc file for the move script.
  • frameworks/base/services/core/java/com/android/server/pm/OtaDexoptService.java (plus OtaDexoptShellCommand) is the package manager that prepares dex2oat commands for applications.

For a working example, refer to /device/google/marlin/device-common.mk.

Update engine logs

For Android 8.x releases and earlier, the update_engine logs can be found in logcat and in the bug report. To make the update_engine logs available in the file system, patch the following changes into your build:

These changes save a copy of the most recent update_engine log to /data/misc/update_engine_log/update_engine.YEAR-TIME. In addition to the current log, the five most recent logs are saved under /data/misc/update_engine_log/. Users with the log group ID will be able to access the file system logs.

Bootloader interactions

The boot_control HAL is used by update_engine (and possibly other daemons) to instruct the bootloader what to boot from. Common example scenarios and their associated states include the following:

  • Normal case: The system is running from its current slot, either slot A or B. No updates have been applied so far. The system's current slot is bootable, successful, and the active slot.
  • Update in progress: The system is running from slot B, so slot B is the bootable, successful, and active slot. Slot A was marked as unbootable since the contents of slot A are being updated but not yet completed. A reboot in this state should continue booting from slot B.
  • Update applied, reboot pending: The system is running from slot B, slot B is bootable and successful, but slot A was marked as active (and therefore is marked as bootable). Slot A is not yet marked as successful and some number of attempts to boot from slot A should be made by the bootloader.
  • System rebooted into new update: The system is running from slot A for the first time, slot B is still bootable and successful while slot A is only bootable, and still active but not successful. A user space daemon, update_verifier, should mark slot A as successful after some checks are made.

Streaming update support

User devices don't always have enough space on /data to download the update package. As neither OEMs nor users want to waste space on a /cache partition, some users go without updates because the device has nowhere to store the update package. To address this issue, Android 8.0 added support for streaming A/B updates that write blocks directly to the B partition as they are downloaded, without having to store the blocks on /data. Streaming A/B updates need almost no temporary storage and require just enough storage for roughly 100 KiB of metadata.

To enable streaming updates in Android 7.1, cherrypick the following patches:

These patches are required to support streaming A/B updates in Android 7.1 and later whether using Google Mobile Services (GMS) or any other update client.

Life of an A/B update

The update process starts when an OTA package (referred to in code as a payload) is available for downloading. Policies in the device may defer the payload download and application based on battery level, user activity, charging status, or other policies. In addition, because the update runs in the background, users might not know an update is in progress. All of this means the update process might be interrupted at any point due to policies, unexpected reboots, or user actions.

Optionally, metadata in the OTA package itself indicates the update can be streamed; the same package can also be used for non-streaming installation. The server may use the metadata to tell the client it's streaming so the client will hand off the OTA to update_engine correctly. Device manufacturers with their own server and client can enable streaming updates by ensuring the server identifies the update is streaming (or assumes all updates are streaming) and the client makes the correct call to update_engine for streaming. Manufacturers can use the fact that the package is of the streaming variant to send a flag to the client to trigger hand off to the framework side as streaming.

After a payload is available, the update process is as follows:

StepActivities
1The current slot (or 'source slot') is marked as successful (if not already marked) with markBootSuccessful().
2 The unused slot (or 'target slot') is marked as unbootable by calling the function setSlotAsUnbootable(). The current slot is always marked as successful at the beginning of the update to prevent the bootloader from falling back to the unused slot, which will soon have invalid data. If the system has reached the point where it can start applying an update, the current slot is marked as successful even if other major components are broken (such as the UI in a crash loop) as it is possible to push new software to fix these problems.
The update payload is an opaque blob with the instructions to update to the new version. The update payload consists of the following:
  • Metadata. A relatively small portion of the update payload, the metadata contains a list of operations to produce and verify the new version on the target slot. For example, an operation could decompress a certain blob and write it to specific blocks in a target partition, or read from a source partition, apply a binary patch, and write to certain blocks in a target partition.
  • Extra data. As the bulk of the update payload, the extra data associated with the operations consists of the compressed blob or binary patch in these examples.
3The payload metadata is downloaded.
4 For each operation defined in the metadata, in order, the associated data (if any) is downloaded to memory, the operation is applied, and the associated memory is discarded.
5 The whole partitions are re-read and verified against the expected hash.
6 The post-install step (if any) is run. In the case of an error during the execution of any step, the update fails and is re-attempted with possibly a different payload. If all the steps so far have succeeded, the update succeeds and the last step is executed.
7 The unused slot is marked as active by calling setActiveBootSlot(). Marking the unused slot as active doesn't mean it will finish booting. The bootloader (or system itself) can switch the active slot back if it doesn't read a successful state.
8 Post-installation (described below) involves running a program from the 'new update' version while still running in the old version. If defined in the OTA package, this step is mandatory and the program must return with exit code 0; otherwise, the update fails.
9 After the system successfully boots far enough into the new slot and finishes the post-reboot checks, the now current slot (formerly the 'target slot') is marked as successful by calling markBootSuccessful().
Note: Steps 3 and 4 take most of the update time as they involve writing and downloading large amounts of data, and are likely to be interrupted for reasons of policy or reboot.

Post-installation

For every partition where a post-install step is defined, update_engine mounts the new partition into a specific location and executes the program specified in the OTA relative to the mounted partition. For example, if the post-install program is defined as usr/bin/postinstall in the system partition, this partition from the unused slot will be mounted in a fixed location (such as /postinstall_mount) and the /postinstall_mount/usr/bin/postinstall command is executed.

For post-installation to succeed, the old kernel must be able to:

  • Mount the new filesystem format. The filesystem type cannot change unless there's support for it in the old kernel, including details such as the compression algorithm used if using a compressed filesystem (i.e. SquashFS).
  • Understand the new partition's post-install program format. If using an Executable and Linkable Format (ELF) binary, it should be compatible with the old kernel (e.g. a 64-bit new program running on an old 32-bit kernel if the architecture switched from 32- to 64-bit builds). Unless the loader (ld) is instructed to use other paths or build a static binary, libraries will be loaded from the old system image and not the new one.

For example, you could use a shell script as a post-install program interpreted by the old system's shell binary with a #! marker at the top), then set up library paths from the new environment for executing a more complex binary post-install program. Alternatively, you could run the post-install step from a dedicated smaller partition to enable the filesystem format in the main system partition to be updated without incurring backward compatibility issues or stepping-stone updates; this would allow users to update directly to the latest version from a factory image.

The new post-install program is limited by the SELinux policies defined in the old system. As such, the post-install step is suitable for performing tasks required by design on a given device or other best-effort tasks (i.e. updating the A/B-capable firmware or bootloader, preparing copies of databases for the new version, etc.). The post-install step is not suitable for one-off bug fixes before reboot that require unforeseen permissions.

The selected post-install program runs in the postinstall SELinux context. All the files in the new mounted partition will be tagged with postinstall_file, regardless of what their attributes are after rebooting into that new system. Changes to the SELinux attributes in the new system won't impact the post-install step. If the post-install program needs extra permissions, those must be added to the post-install context.

After reboot

After rebooting, update_verifier triggers the integrity check using dm-verity. This check starts before zygote to avoid Java services making any irreversible changes that would prevent a safe rollback. During this process, bootloader and kernel may also trigger a reboot if verified boot or dm-verity detect any corruption. After the check completes, update_verifier marks the boot successful.

update_verifier will read only the blocks listed in /data/ota_package/care_map.txt, which is included in an A/B OTA package when using the AOSP code. The Java system update client, such as GmsCore, extracts care_map.txt, sets up the access permission before rebooting the device, and deletes the extracted file after the system successfully boots into the new version.

  • Operating System Tutorial
  • OS - Exams Questions with Answers
  • Operating System Useful Resources
  • Selected Reading

Memory management is the functionality of an operating system which handles or manages primary memory and moves processes back and forth between main memory and disk during execution. Memory management keeps track of each and every memory location, regardless of either it is allocated to some process or it is free. It checks how much memory is to be allocated to processes. It decides which process will get memory at what time. It tracks whenever some memory gets freed or unallocated and correspondingly it updates the status.

This tutorial will teach you basic concepts related to Memory Management.

Process Address Space

The process address space is the set of logical addresses that a process references in its code. For example, when 32-bit addressing is in use, addresses can range from 0 to 0x7fffffff; that is, 2^31 possible numbers, for a total theoretical size of 2 gigabytes.

The operating system takes care of mapping the logical addresses to physical addresses at the time of memory allocation to the program. There are three types of addresses used in a program before and after memory is allocated −

S.N.Memory Addresses & Description
1

Symbolic addresses

The addresses used in a source code. The variable names, constants, and instruction labels are the basic elements of the symbolic address space.

2

Relative addresses

At the time of compilation, a compiler converts symbolic addresses into relativeaddresses.

3

Physical addresses

The loader generates these addresses at the time when a program is loaded into main memory.

Virtual and physical addresses are the same in compile-time and load-time address-binding schemes. Virtual and physical addresses differ in execution-time address-binding scheme.

The set of all logical addresses generated by a program is referred to as a logical address space. The set of all physical addresses corresponding to these logical addresses is referred to as a physical address space.

The runtime mapping from virtual to physical address is done by the memory management unit (MMU) which is a hardware device. MMU uses following mechanism to convert virtual address to physical address.

  • The value in the base register is added to every address generated by a user process, which is treated as offset at the time it is sent to memory. For example, if the base register value is 10000, then an attempt by the user to use address location 100 will be dynamically reallocated to location 10100.

  • The user program deals with virtual addresses; it never sees the real physical addresses.

Static vs Dynamic Loading

The choice between Static or Dynamic Loading is to be made at the time of computer program being developed. If you have to load your program statically, then at the time of compilation, the complete programs will be compiled and linked without leaving any external program or module dependency. The linker combines the object program with other necessary object modules into an absolute program, which also includes logical addresses.

If you are writing a Dynamically loaded program, then your compiler will compile the program and for all the modules which you want to include dynamically, only references will be provided and rest of the work will be done at the time of execution.

At the time of loading, with static loading, the absolute program (and data) is loaded into memory in order for execution to start.

If you are using dynamic loading, dynamic routines of the library are stored on a disk in relocatable form and are loaded into memory only when they are needed by the program.

Static vs Dynamic Linking

As explained above, when static linking is used, the linker combines all other modules needed by a program into a single executable program to avoid any runtime dependency.

When dynamic linking is used, it is not required to link the actual module or library with the program, rather a reference to the dynamic module is provided at the time of compilation and linking. Dynamic Link Libraries (DLL) in Windows and Shared Objects in Unix are good examples of dynamic libraries.

Swapping

Swapping is a mechanism in which a process can be swapped temporarily out of main memory (or move) to secondary storage (disk) and make that memory available to other processes. At some later time, the system swaps back the process from the secondary storage to main memory.

Though performance is usually affected by swapping process but it helps in running multiple and big processes in parallel and that's the reason Swapping is also known as a technique for memory compaction.

The total time taken by swapping process includes the time it takes to move the entire process to a secondary disk and then to copy the process back to memory, as well as the time the process takes to regain main memory.

Let us assume that the user process is of size 2048KB and on a standard hard disk where swapping will take place has a data transfer rate around 1 MB per second. The actual transfer of the 1000K process to or from memory will take

Now considering in and out time, it will take complete 4000 milliseconds plus other overhead where the process competes to regain main memory.

Memory Allocation

Main memory usually has two partitions −

  • Low Memory − Operating system resides in this memory.

  • High Memory − User processes are held in high memory.

Operating system uses the following memory allocation mechanism.

S.N.Memory Allocation & Description
1

Single-partition allocation

In this type of allocation, relocation-register scheme is used to protect user processes from each other, and from changing operating-system code and data. Relocation register contains value of smallest physical address whereas limit register contains range of logical addresses. Each logical address must be less than the limit register.

2

Multiple-partition allocation

In this type of allocation, main memory is divided into a number of fixed-sizedpartitions where each partition should contain only one process. When a partitionis free, a process is selected from the input queue and is loaded into the freepartition. When the process terminates, the partition becomes available foranother process.

There Are Not Enough Slots Available In The System To Satisfy New

Fragmentation

As processes are loaded and removed from memory, the free memory space is broken into little pieces. It happens after sometimes that processes cannot be allocated to memory blocks considering their small size and memory blocks remains unused. This problem is known as Fragmentation.

Fragmentation is of two types −

S.N.Fragmentation & Description
1

External fragmentation

Total memory space is enough to satisfy a request or to reside a process in it, but it is not contiguous, so it cannot be used.

2

Internal fragmentation

Memory block assigned to process is bigger. Some portion of memory is left unused, as it cannot be used by another process.

The following diagram shows how fragmentation can cause waste of memory and a compaction technique can be used to create more free memory out of fragmented memory −

External fragmentation can be reduced by compaction or shuffle memory contents to place all free memory together in one large block. To make compaction feasible, relocation should be dynamic.

The internal fragmentation can be reduced by effectively assigning the smallest partition but large enough for the process.

Paging

A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard that's set up to emulate the computer's RAM. Paging technique plays an important role in implementing virtual memory.

Paging is a memory management technique in which process address space is broken into blocks of the same size called pages (size is power of 2, between 512 bytes and 8192 bytes). The size of the process is measured in the number of pages.

Similarly, main memory is divided into small fixed-sized blocks of (physical) memory called frames and the size of a frame is kept the same as that of a page to have optimum utilization of the main memory and to avoid external fragmentation.

Address Translation

Page address is called logical address and represented by page number and the offset.

Frame address is called physical address and represented by a frame number and the offset.

A data structure called page map table is used to keep track of the relation between a page of a process to a frame in physical memory.

Best

When the system allocates a frame to any page, it translates this logical address into a physical address and create entry into the page table to be used throughout execution of the program.

When a process is to be executed, its corresponding pages are loaded into any available memory frames. Suppose you have a program of 8Kb but your memory can accommodate only 5Kb at a given point in time, then the paging concept will come into picture. When a computer runs out of RAM, the operating system (OS) will move idle or unwanted pages of memory to secondary memory to free up RAM for other processes and brings them back when needed by the program.

This process continues during the whole execution of the program where the OS keeps removing idle pages from the main memory and write them onto the secondary memory and bring them back when required by the program.

Advantages and Disadvantages of Paging

Here is a list of advantages and disadvantages of paging −

  • Paging reduces external fragmentation, but still suffer from internal fragmentation.

  • Paging is simple to implement and assumed as an efficient memory management technique.

  • Due to equal size of the pages and frames, swapping becomes very easy.

  • Page table requires extra memory space, so may not be good for a system having small RAM.

Horovod There Are Not Enough Slots Available In The System To Satisfy

Segmentation

Segmentation is a memory management technique in which each job is divided into several segments of different sizes, one for each module that contains pieces that perform related functions. Each segment is actually a different logical address space of the program.

When a process is to be executed, its corresponding segmentation are loaded into non-contiguous memory though every segment is loaded into a contiguous block of available memory.

Segmentation memory management works very similar to paging but here segments are of variable-length where as in paging pages are of fixed size.

There Are Not Enough Slots Available In The System To Satisfy Free

A program segment contains the program's main function, utility functions, data structures, and so on. The operating system maintains a segment map table for every process and a list of free memory blocks along with segment numbers, their size and corresponding memory locations in main memory. For each segment, the table stores the starting address of the segment and the length of the segment. A reference to a memory location includes a value that identifies a segment and an offset.