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Full Discussion: Physical RAM
Operating Systems Linux Red Hat Physical RAM Post 302621697 by sds9985 on Tuesday 10th of April 2012 11:35:37 PM
Old 04-11-2012
An active RHEL system should be using almost all of the free physical memory for cache and buffers. Once you start doing application I/O, you'll see the memory get "used" by the OS quickly. Remember, the memory "used" for cache/buffers is actually free and the kernel can reclaim it very quickly if memory demand for processes increases. The "free" command shows you the difference between "used" memory with and without including the cache/buffers.

Older versions of Linux did have a penalty for very large memory because the algorithms used to manage memory pages were primitive. Many of those problems were addressed in later RHEL 4 releases and by the time we got to RHEL 5 and 6, those issues have been completely resolved. Many of our servers run 256 or 512GB of RAM. For a BL460c with maybe 8 cores, 32GB seems to me to be about right. If it's a BL460cG6 with 16 cores, 32GB seems small. It all depends on what applications or databases you're running on it.

Here's an example from a mid range system with 48 cores:

Code:
# free -g
           total       used       free     shared    buffers     cached
Mem:           251        250          0          0          0        226
-/+ buffers/cache:         24        226
Swap:          294         20        273

At first glance it looks like this system has no free memory. But the actual memory used by processes is actually only 24GB - the other 226GB is cache/buffers.

With very large amounts of RAM on certain hardware you can encounter NUMA issues, but as a general rule, you can never have too much memory. RHEL will find a way to use it.
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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)
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