06-16-2010
The performance impact is highly workload dependent, but on a fairly typical kernel compile,the PAE penalty works out to be around a 1% performance hit on RedHat’s test boxes.Testing with various other workload mixes has given performance
hits ranging from 0% to 10%.
From Redhat's 2.1 Virtual memory white paper. Things may have changed slightly in 2.6, but as such, it will still produce a slight performance hit.
Last edited by mark54g; 06-16-2010 at 10:52 PM..
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LEARN ABOUT OPENSOLARIS
bcopy
bcopy(9F) Kernel Functions for Drivers bcopy(9F)
NAME
bcopy - copy data between address locations in the kernel
SYNOPSIS
#include <sys/types.h>
#include <sys/sunddi.h>
void bcopy(const void *from, void *to, size_t bcount);
INTERFACE LEVEL
Architecture independent level 1 (DDI/DKI).
PARAMETERS
from Source address from which the copy is made.
to Destination address to which copy is made.
bcount The number of bytes moved.
DESCRIPTION
The bcopy() function copies bcount bytes from one kernel address to another. If the input and output addresses overlap, the command exe-
cutes, but the results may not be as expected.
Note that bcopy() should never be used to move data in or out of a user buffer, because it has no provision for handling page faults. The
user address space can be swapped out at any time, and bcopy() always assumes that there will be no paging faults. If bcopy() attempts to
access the user buffer when it is swapped out, the system will panic. It is safe to use bcopy() to move data within kernel space, since
kernel space is never swapped out.
CONTEXT
The bcopy() function can be called from user, interrupt, or kernel context.
EXAMPLES
Example 1 Copying data between address locations in the kernel:
An I/O request is made for data stored in a RAM disk. If the I/O operation is a read request, the data is copied from the RAM disk to a
buffer (line 8). If it is a write request, the data is copied from a buffer to the RAM disk (line 15). bcopy() is used since both the RAM
disk and the buffer are part of the kernel address space.
1 #define RAMDNBLK 1000 /* blocks in the RAM disk */
2 #define RAMDBSIZ 512 /* bytes per block */
3 char ramdblks[RAMDNBLK][RAMDBSIZ]; /* blocks forming RAM
/* disk
...
4
5 if (bp->b_flags & B_READ) /* if read request, copy data */
6 /* from RAM disk data block */
7 /* to system buffer */
8 bcopy(&ramdblks[bp->b_blkno][0], bp->b_un.b_addr,
9 bp->b_bcount);
10
11 else /* else write request, */
12 /* copy data from a */
13 /* system buffer to RAM disk */
14 /* data block */
15 bcopy(bp->b_un.b_addr, &ramdblks[bp->b_blkno][0],
16 bp->b_bcount);
SEE ALSO
copyin(9F), copyout(9F)
Writing Device Drivers
WARNINGS
The from and to addresses must be within the kernel space. No range checking is done. If an address outside of the kernel space is
selected, the driver may corrupt the system in an unpredictable way.
SunOS 5.11 16 Jan 2006 bcopy(9F)