Specifications

This is the Title of the Book, eMatter Edition

Performing Direct I/O

435

Performing Direct I/O

Most I/O operations are buffered through the kernel. The use of a kernel-space

buffer allows a degree of separation between user space and the actual device; this

separation can make programming easier and can also yield performance benefits in

many situations. There are cases, however, where it can be beneficial to perform I/O

directly to or from a user-space buffer. If the amount of data being transferred is

large, transferring data directly without an extra copy through kernel space can

speed things up.

One example of direct I/O use in the 2.6 kernel is the SCSI tape driver. Streaming

tapes can pass a lot of data through the system, and tape transfers are usually record-

oriented, so there is little benefit to buffering data in the kernel. So, when the condi-

tions are right (the user-space buffer is page-aligned, for example), the SCSI tape

driver performs its I/O without copying the data.

That said, it is important to recognize that direct I/O does not always provide the

performance boost that one might expect. The overhead of setting up direct I/O

(which involves faulting in and pinning down the relevant user pages) can be signifi-

cant, and the benefits of buffered I/O are lost. For example, the use of direct I/O

requires that the write system call operate synchronously; otherwise the application

does not know when it can reuse its I/O buffer. Stopping the application until each

write completes can slow things down, which is why applications that use direct I/O

often use asynchronous I/O operations as well.

The real moral of the story, in any case, is that implementing direct I/O in a char

driver is usually unnecessary and can be hurtful. You should take that step only if

you are sure that the overhead of buffered I/O is truly slowing things down. Note

also that block and network drivers need not worry about implementing direct I/O at

all; in both cases, higher-level code in the kernel sets up and makes use of direct I/O

when it is indicated, and driver-level code need not even know that direct I/O is

being performed.

The key to implementing direct I/O in the 2.6 kernel is a function called get_user_pages,

which is declared in <linux/mm.h> with the following prototype:

int get_user_pages(struct task_struct *tsk,

struct mm_struct *mm,

unsigned long start,

int len,

int write,

int force,

struct page **pages,

struct vm_area_struct **vmas);

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