Linux I/O Models

http://www.ibm.com/developerworks/library/l-async/

The most common input/output (I/O) model used in Linux® is synchronous I/O. After a request is made in this model, the application blocks until the request is satisfied. This is a great paradigm because the calling application requires no central processing unit (CPU) while it awaits the completion of the I/O request. But in some cases there's a need to overlap an I/O request with other processing. The Portable Operating System Interface (POSIX) asynchronous I/O (AIO) application program interface (API) provides this capability. In this article, get an overview of the API and see how to use it.

Introduction to AIO

Linux asynchronous I/O is a relatively recent addition to the Linux kernel. It's a standard feature of the 2.6 kernel, but you can find patches for 2.4. The basic idea behind AIO is to allow a process to initiate a number of I/O operations without having to block or wait for any to complete. At some later time, or after being notified of I/O completion, the process can retrieve the results of the I/O.

I/O models

Before digging into the AIO API, let's explore the different I/O models that are available under Linux. This isn't intended as an exhaustive review, but rather aims to cover the most common models to illustrate their differences from asynchronous I/O. Figure 1 shows synchronous and asynchronous models, as well as blocking and non-blocking models.

Figure 1. Simplified matrix of basic Linux I/O models

Each of these I/O models has usage patterns that are advantageous for particular applications. This section briefly explores each one.

Synchronous blocking I/O

I/O-bound versus CPU-bound processes

A process that is I/O bound is one that performs more I/O than processing. A CPU-bound process does more processing than I/O. The Linux 2.6 scheduler actually favors I/O-bound processes because they commonly initiate an I/O and then block, which means other work can be efficiently interlaced between them.

One of the most common models is the synchronous blocking I/O model. In this model, the user-space application performs a system call that results in the application blocking. This means that the application blocks until the system call is complete (data transferred or error). The calling application is in a state where it consumes no CPU and simply awaits the response, so it is efficient from a processing perspective.

Figure 2 illustrates the traditional blocking I/O model, which is also the most common model used in applications today. Its behaviors are well understood, and its usage is efficient for typical applications. When the 

read

 system call is invoked, the application blocks and the context switches to the kernel. The read is then initiated, and when the response returns (from the device from which you're reading), the data is moved to the user-space buffer. Then the application is unblocked (and the 

read

 call returns).

Figure 2. Typical flow of the synchronous blocking I/O model

From the application's perspective, the 

read

 call spans a long duration. But, in fact, the application is actually blocked while the read is multiplexed with other work in the kernel.

Synchronous non-blocking I/O

A less efficient variant of synchronous blocking is synchronous non-blocking I/O. In this model, a device is opened as non-blocking. This means that instead of completing an I/O immediately, a 

read

 may return an error code indicating that the command could not be immediately satisfied (

EAGAIN

 or 

EWOULDBLOCK

), as shown in Figure 3.

Figure 3. Typical flow of the synchronous non-blocking I/O model

The implication of non-blocking is that an I/O command may not be satisfied immediately, requiring that the application make numerous calls to await completion. This can be extremely inefficient because in many cases the application must busy-wait until the data is available or attempt to do other work while the command is performed in the kernel. As also shown in Figure 3, this method can introduce latency in the I/O because any gap between the data becoming available in the kernel and the user calling 

read

 to return it can reduce the overall data throughput.

Asynchronous blocking I/O

Another blocking paradigm is non-blocking I/O with blocking notifications. In this model, non-blocking I/O is configured, and then the blocking

select

 system call is used to determine when there's any activity for an I/O descriptor. What makes the 

select

 call interesting is that it can be used to provide notification for not just one descriptor, but many. For each descriptor, you can request notification of the descriptor's ability to write data, availability of read data, and also whether an error has occurred.

Figure 4. Typical flow of the asynchronous blocking I/O model (select)

The primary issue with the 

select

 call is that it's not very efficient. While it's a convenient model for asynchronous notification, its use for high-performance I/O is not advised.

Asynchronous non-blocking I/O (AIO)

Finally, the asynchronous non-blocking I/O model is one of overlapping processing with I/O. The read request returns immediately, indicating that the 

read

 was successfully initiated. The application can then perform other processing while the background read operation completes. When the

read

 response arrives, a signal or a thread-based callback can be generated to complete the I/O transaction.

Figure 5. Typical flow of the asynchronous non-blocking I/O model

The ability to overlap computation and I/O processing in a single process for potentially multiple I/O requests exploits the gap between processing speed and I/O speed. While one or more slow I/O requests are pending, the CPU can perform other tasks or, more commonly, operate on already completed I/Os while other I/Os are initiated.

The next section examines this model further, explores the API, and then demonstrates a number of the commands.

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Motivation for asynchronous I/O

From the previous taxonomy of I/O models, you can see the motivation for AIO. The blocking models require the initiating application to block when the I/O has started. This means that it isn't possible to overlap processing and I/O at the same time. The synchronous non-blocking model allows overlap of processing and I/O, but it requires that the application check the status of the I/O on a recurring basis. This leaves asynchronous non-blocking I/O, which permits overlap of processing and I/O, including notification of I/O completion.

The functionality provided by the 

select

 function (asynchronous blocking I/O) is similar to AIO, except that it still blocks. However, it blocks on notifications instead of the I/O call.

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Introduction to AIO for Linux

This section explores the asynchronous I/O model for Linux to help you understand how to apply it in your applications.

In a traditional I/O model, there is an I/O channel that is identified by a unique handle. In UNIX®, these are file descriptors (which are the same for files, pipes, sockets, and so on). In blocking I/O, you initiate a transfer and the system call returns when it's complete or an error has occurred.

AIO for Linux

AIO first entered the Linux kernel in 2.5 and is now a standard feature of 2.6 production kernels.

In asynchronous non-blocking I/O, you have the ability to initiate multiple transfers at the same time. This requires a unique context for each transfer so you can identify it when it completes. In AIO, this is an 

aiocb

 (AIO I/O Control Block) structure. This structure contains all of the information about a transfer, including a user buffer for data. When notification for an I/O occurs (called a completion), the 

aiocb

 structure is provided to uniquely identify the completed I/O. The API demonstration shows how to do this.

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AIO API

The AIO interface API is quite simple, but it provides the necessary functions for data transfer with a couple of different notification models. Table 1 shows the AIO interface functions, which are further explained later in this section.

Table 1. AIO interface APIs

API function	Description
aio_read	Request an asynchronous read operation
aio_error	Check the status of an asynchronous request
aio_return	Get the return status of a completed asynchronous request
aio_write	Request an asynchronous operation
aio_suspend	Suspend the calling process until one or more asynchronous requests have completed (or failed)
aio_cancel	Cancel an asynchronous I/O request
lio_listio	Initiate a list of I/O operations

Each of these API functions uses the 

aiocb

 structure for initiating or checking. This structure has a number of elements, but Listing 1 shows only the ones that you'll need to (or can) use.

Listing 1. The aiocb structure showing the relevant fields

struct aiocb {

  int aio_fildes;               // File Descriptor
  int aio_lio_opcode;           // Valid only for lio_listio (r/w/nop)
  volatile void *aio_buf;       // Data Buffer
  size_t aio_nbytes;            // Number of Bytes in Data Buffer
  struct sigevent aio_sigevent; // Notification Structure

  /* Internal fields */
  ...

};      

The 

sigevent

 structure tells AIO what to do when the I/O completes. You'll explore this structure in the AIO demonstration. Now I'll show you how the individual API functions for AIO work and how you can use them.

aio_read

The 

aio_read

 function requests an asynchronous read operation for a valid file descriptor. The file descriptor can represent a file, a socket, or even a pipe. The 

aio_read

 function has the following prototype:

int aio_read( struct aiocb *aiocbp );      

The 

aio_read

 function returns immediately after the request has been queued. The return value is zero on success or -1 on error, where 

errno

is defined.

To perform a read, the application must initialize the 

aiocb

 structure. The following short example illustrates filling in the 

aiocb

 request structure and using 

aio_read

 to perform an asynchronous read request (ignore notification for now). It also shows use of the 

aio_error

 function, but I'll explain that later.

Listing 2. Sample code for an asynchronous read with aio_read

#include <aio.h>

...

  int fd, ret;
  struct aiocb my_aiocb;

  fd = open( "file.txt", O_RDONLY );
  if (fd < 0) perror("open");

  /* Zero out the aiocb structure (recommended) */
  bzero( (char *)&my_aiocb, sizeof(struct aiocb) );

  /* Allocate a data buffer for the aiocb request */
  my_aiocb.aio_buf = malloc(BUFSIZE+1);
  if (!my_aiocb.aio_buf) perror("malloc");

  /* Initialize the necessary fields in the aiocb */
  my_aiocb.aio_fildes = fd;
  my_aiocb.aio_nbytes = BUFSIZE;
  my_aiocb.aio_offset = 0;

  ret = aio_read( &my_aiocb );
  if (ret < 0) perror("aio_read");

  while ( aio_error( &my_aiocb ) == EINPROGRESS ) ;

  if ((ret = aio_return( &my_iocb )) > 0) {
    /* got ret bytes on the read */
  } else {
    /* read failed, consult errno */
  }      

In Listing 2, after the file from which you're reading data is opened, you zero out your 

aiocb

 structure, and then allocate a data buffer. The reference to the data buffer is placed into 

aio_buf

. Subsequently, you initialize the size of the buffer into 

aio_nbytes

. The 

aio_offset

 is set to zero (the first offset in the file). You set the file descriptor from which you're reading into 

aio_fildes

. After these fields are set, you call

aio_read

 to request the read. You can then make a call to 

aio_error

 to determine the status of the 

aio_read

. As long as the status is

EINPROGRESS

, you busy-wait until the status changes. At this point, your request has either succeeded or failed.

Building with the AIO interface

You can find the function prototypes and other necessary symbolics in the 

aio.h

 header file. When building an application that uses this interface, you must use the POSIX real-time extensions library (

librt

).

Note the similarities to reading from the file with the standard library functions. In addition to the asynchronous nature of 

aio_read

, another difference is setting the offset for the read. In a typical 

read

 call, the offset is maintained for you in the file descriptor context. For each read, the offset is updated so that subsequent reads address the next block of data. This isn't possible with asynchronous I/O because you can perform many read requests simultaneously, so you must specify the offset for each particular read request.

aio_error

The 

aio_error

 function is used to determine the status of a request. Its prototype is:

int aio_error( struct aiocb *aiocbp );      

This function can return the following:

• EINPROGRESS

  , indicating the request has not yet completed

• ECANCELLED

  , indicating the request was cancelled by the application

• -1

  , indicating that an error occurred for which you can consult 

  errno

aio_return

Another difference between asynchronous I/O and standard blocking I/O is that you don't have immediate access to the return status of your function because you're not blocking on the 

read

 call. In a standard 

read

 call, the return status is provided upon return of the function. With asynchronous I/O, you use the 

aio_return

 function. This function has the following prototype:

ssize_t aio_return( struct aiocb *aiocbp );      

This function is called only after the 

aio_error

 call has determined that your request has completed (either successfully or in error). The return value of 

aio_return

 is identical to that of the 

read

 or 

write

 system call in a synchronous context (number of bytes transferred or 

-1

 for error).

aio_write

The 

aio_write

 function is used to request an asynchronous write. Its function prototype is:

int aio_write( struct aiocb *aiocbp );      

The 

aio_write

 function returns immediately, indicating that the request has been enqueued (with a return of 

 on success and 

-1

 on failure, with

errno

 properly set).

This is similar to the 

read

 system call, but one behavior difference is worth noting. Recall that the offset to be used is important with the 

read

 call. However, with 

write

, the offset is important only if used in a file context where the 

O_APPEND

 option is not set. If 

O_APPEND

 is set, then the offset is ignored and the data is appended to the end of the file. Otherwise, the 

aio_offset

 field determines the offset at which the data is written to the file.

aio_suspend

You can use the 

aio_suspend

 function to suspend (or block) the calling process until an asynchronous I/O request has completed, a signal is raised, or an optional timeout occurs. The caller provides a list of 

aiocb

 references for which the completion of at least one will cause

aio_suspend

 to return. The function prototype for 

aio_suspend

 is:

int aio_suspend( const struct aiocb *const cblist[],
                  int n, const struct timespec *timeout );      

Using 

aio_suspend

 is quite simple. A list of 

aiocb

 references is provided. If any of them complete, the call returns with 

. Otherwise, 

-1

 is returned, indicating an error occurred. See Listing 3.

Listing 3. Using the aio_suspend function to block on asynchronous I/Os

struct aioct *cblist[MAX_LIST]

/* Clear the list. */
bzero( (char *)cblist, sizeof(cblist) );

/* Load one or more references into the list */
cblist[0] = &my_aiocb;

ret = aio_read( &my_aiocb );

ret = aio_suspend( cblist, MAX_LIST, NULL );      

Note that the second argument of 

aio_suspend

 is the number of elements in 

cblist

, not the number of 

aiocb

 references. Any 

NULL

 element in the 

cblist

 is ignored by 

aio_suspend

.

If a timeout is provided to 

aio_suspend

 and the timeout occurs, then 

-1

is returned and 

errno

 contains 

EAGAIN

.

aio_cancel

The 

aio_cancel

 function allows you to cancel one or all outstanding I/O requests for a given file descriptor. Its prototype is:

int aio_cancel( int fd, struct aiocb *aiocbp );      

To cancel a single request, provide the file descriptor and the 

aiocb

 reference. If the request is successfully cancelled, the function returns

AIO_CANCELED

. If the request completes, the function returns 

AIO_NOTCANCELED

.

To cancel all requests for a given file descriptor, provide that file descriptor and a 

NULL

 reference for 

aiocbp

. The function returns

AIO_CANCELED

 if all requests are canceled, 

AIO_NOT_CANCELED

 if at least one request couldn't be canceled, and 

AIO_ALLDONE

 if none of the requests could be canceled. You can then evaluate each individual AIO request using 

aio_error

. If the request was canceled, 

aio_error

returns 

-1

, and 

errno

 is set to 

ECANCELED

.

lio_listio

Finally, AIO provides a way to initiate multiple transfers at the same time using the 

lio_listio

 API function. This function is important because it means you can start lots of I/Os in the context of a single system call (meaning one kernel context switch). This is great from a performance perspective, so it's worth exploring. The 

lio_listio

 API function has the following prototype:

int lio_listio( int mode, struct aiocb *list[], int nent,
                   struct sigevent *sig );      

The 

mode

 argument can be 

LIO_WAIT

 or 

LIO_NOWAIT

. 

LIO_WAIT

 blocks the call until all I/O has completed. 

LIO_NOWAIT

 returns after the operations have been queued. The 

list

 is a list of 

aiocb

 references, with the maximum number of elements defined by 

nent

. Note that elements of 

list

 may be 

NULL

, which 

lio_listio

 ignores. The 

sigevent

 reference defines the method for signal notification when all I/O is complete.

The request for 

lio_listio

 is slightly different than the typical 

read

 or 

write

 request in that the operation must be specified. This is illustrated in Listing 4.

Listing 4. Using the lio_listio function to initiate a list of requests

struct aiocb aiocb1, aiocb2;
struct aiocb *list[MAX_LIST];

...

/* Prepare the first aiocb */
aiocb1.aio_fildes = fd;
aiocb1.aio_buf = malloc( BUFSIZE+1 );
aiocb1.aio_nbytes = BUFSIZE;
aiocb1.aio_offset = next_offset;
aiocb1.aio_lio_opcode = LIO_READ;

...

bzero( (char *)list, sizeof(list) );
list[0] = &aiocb1;
list[1] = &aiocb2;

ret = lio_listio( LIO_WAIT, list, MAX_LIST, NULL );      

The read operation is noted in the 

aio_lio_opcode

 field with 

LIO_READ

. For a write operation, 

LIO_WRITE

 is used, but 

LIO_NOP

 is also valid for no operation.

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AIO notifications

Now that you've seen the AIO functions that are available, this section digs into the methods that you can use for asynchronous notification. I'll explore asynchronous notification through signals and function callbacks.

Asynchronous notification with signals

The use of signals for interprocess communication (IPC) is a traditional mechanism in UNIX and is also supported by AIO. In this paradigm, the application defines a signal handler that is invoked when a specified signal occurs. The application then specifies that an asynchronous request will raise a signal when the request has completed. As part of the signal context, the particular 

aiocb

 request is provided to keep track of multiple potentially outstanding requests. Listing 5 demonstrates this notification method.

Listing 5. Using signals as notification for AIO requests

void setup_io( ... )
{
  int fd;
  struct sigaction sig_act;
  struct aiocb my_aiocb;

  ...

  /* Set up the signal handler */
  sigemptyset(&sig_act.sa_mask);
  sig_act.sa_flags = SA_SIGINFO;
  sig_act.sa_sigaction = aio_completion_handler;


  /* Set up the AIO request */
  bzero( (char *)&my_aiocb, sizeof(struct aiocb) );
  my_aiocb.aio_fildes = fd;
  my_aiocb.aio_buf = malloc(BUF_SIZE+1);
  my_aiocb.aio_nbytes = BUF_SIZE;
  my_aiocb.aio_offset = next_offset;

  /* Link the AIO request with the Signal Handler */
  my_aiocb.aio_sigevent.sigev_notify = SIGEV_SIGNAL;
  my_aiocb.aio_sigevent.sigev_signo = SIGIO;
  my_aiocb.aio_sigevent.sigev_value.sival_ptr = &my_aiocb;

  /* Map the Signal to the Signal Handler */
  ret = sigaction( SIGIO, &sig_act, NULL );

  ...

  ret = aio_read( &my_aiocb );

}


void aio_completion_handler( int signo, siginfo_t *info, void *context )
{
  struct aiocb *req;


  /* Ensure it's our signal */
  if (info->si_signo == SIGIO) {

    req = (struct aiocb *)info->si_value.sival_ptr;

    /* Did the request complete? */
    if (aio_error( req ) == 0) {

      /* Request completed successfully, get the return status */
      ret = aio_return( req );

    }

  }

  return;
}      

In Listing 5, you set up your signal handler to catch the 

SIGIO

 signal in the 

aio_completion_handler

 function. You then initialize the

aio_sigevent

 structure to raise 

SIGIO

 for notification (which is specified via the 

SIGEV_SIGNAL

 definition in 

sigev_notify

). When your read completes, your signal handler extracts the particular 

aiocb

 from the signal's 

si_value

 structure and checks the error status and return status to determine I/O completion.

For performance, the completion handler is an ideal spot to continue the I/O by requesting the next asynchronous transfer. In this way, when completion of one transfer has completed, you immediately start the next.

Asynchronous notification with callbacks

An alternative notification mechanism is the system callback. Instead of raising a signal for notification, this mechanism calls a function in user-space for notification. You initialize the 

aiocb

 reference into the 

sigevent

 structure to uniquely identify the particular request being completed; see Listing 6.

Listing 6. Using thread callback notification for AIO requests

void setup_io( ... )
{
  int fd;
  struct aiocb my_aiocb;

  ...

  /* Set up the AIO request */
  bzero( (char *)&my_aiocb, sizeof(struct aiocb) );
  my_aiocb.aio_fildes = fd;
  my_aiocb.aio_buf = malloc(BUF_SIZE+1);
  my_aiocb.aio_nbytes = BUF_SIZE;
  my_aiocb.aio_offset = next_offset;

  /* Link the AIO request with a thread callback */
  my_aiocb.aio_sigevent.sigev_notify = SIGEV_THREAD;
  my_aiocb.aio_sigevent.notify_function = aio_completion_handler;
  my_aiocb.aio_sigevent.notify_attributes = NULL;
  my_aiocb.aio_sigevent.sigev_value.sival_ptr = &my_aiocb;

  ...

  ret = aio_read( &my_aiocb );

}


void aio_completion_handler( sigval_t sigval )
{
  struct aiocb *req;

  req = (struct aiocb *)sigval.sival_ptr;

  /* Did the request complete? */
  if (aio_error( req ) == 0) {

    /* Request completed successfully, get the return status */
    ret = aio_return( req );

  }

  return;
}      

In Listing 6, after creating your 

aiocb

 request, you request a thread callback using 

SIGEV_THREAD

 for the notification method. You then specify the particular notification handler and load the context to be passed to the handler (in this case, a reference to the 

aiocb

 request itself). In the handler, you simply cast the incoming 

sigval

 pointer and use the AIO functions to validate the completion of the request.

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System tuning for AIO

The proc file system contains two virtual files that can be tuned for asynchronous I/O performance:

• The /proc/sys/fs/aio-nr file provides the current number of system-wide asynchronous I/O requests.
• The /proc/sys/fs/aio-max-nr file is the maximum number of allowable concurrent requests. The maximum is commonly 64KB, which is adequate for most applications.

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Summary

Using asynchronous I/O can help you build faster and more efficient I/O applications. If your application can overlap processing and I/O, then AIO can help you build an application that more efficiently uses the CPU resources available to you. While this I/O model differs from the traditional blocking patterns found in most Linux applications, the asynchronous notification model is conceptually simple and can simplify your design.