Visit Our UNIX and Linux User Community

Memory and cache access time discrepancy

Thread Tools Search this Thread
Top Forums Programming Memory and cache access time discrepancy
# 1  
Old 12-10-2014
Memory and cache access time discrepancy

#include "rdtsc.h"
#define SIZE 4*64*1024

int main()
unsigned long long a,b;
int arr[SIZE]={0};
int i;
register int r;


printf("1st element Access Cycles = %llu\n",b-a);

printf("2nd Element Access Cycles = %llu\n",b-a);


In the above code I am trying to determine the number of cycles it takes to fetch the first element of an array from memory, and then the next cached element. When I execute this snippet, I am getting almost identical number of cycles for both accesses ~81 cycles. Can anybody explain me why this is happening. By all means, the first access should be very costly, but the access to next sequential element which has been brought into the cache should be much lesser.

# 2  
Old 12-10-2014
Entering main() writes zero values into all elements of arr which might or might not overflow your cache. Even if it does overflow your cache, setting a with your first call to rdtsc() will probably pull in the first several elements of arr since a, b, and arr will probably be allocated close to each other on the stack.
# 3  
Old 12-10-2014
The compiler's optimizer noted that the value of "r" was never used, and eliminated the operation.

And if the code isn't optimized, the compiler almost certainly produced a LOT more instructions than necessary to simply load a value from memory into a register.

Nevermind the time spend actually doing the rdtsc() routines themselves.

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. AIX

AIX swap space, physical memory & cache

Hi, I am new to AIX, Can someone please help me how to know the swap space, total physical memory and system cache? We are using AIX 5.3. Thanks! (3 Replies)
Discussion started by: Phaneendra G
3 Replies

2. UNIX for Dummies Questions & Answers

Clearing memory cache on Linux server

i wish to clear memory cache on a production box and i was wondering what is the worst that can happen if i do? i already tested this on a backup server and everything seemed fine. but i need to know from you experts what are the worst things that can happen when i run it on a real server: ... (5 Replies)
Discussion started by: SkySmart
5 Replies

3. Solaris

clear cache memory

hi all, i have noticed that my server has 64 GB RAM and i have application in this server but the server has free memory only 15% and utilized 85% however it didn't eat from swap . does any parameter can be configured in kernel to make the system clear memory from cache like linux i found... (4 Replies)
Discussion started by: maxim42
4 Replies

4. Programming

Ways to preserve a memory cache

The environment is Java/Windows. The program keeps near real-time state in memory cache, which is updated by multiple sources, size of the cache is roughly 500 MB, frequency of updates is ~ 20 per second. I am looking into different ways to keep current snapshot of the memory on the disk for a)... (9 Replies)
Discussion started by: migurus
9 Replies

5. Shell Programming and Scripting

Discrepancy in finding the top memory consuming processes

When I run 'top' command,I see the following Memory: 32G real, 12G free, 96G swap free Though it shows as 12G free,I am not able to account for processes that consume the rest 20G. In my understanding some process should be consuming atleast 15-16 G but I am not able to find them. Is... (1 Reply)
Discussion started by: prasperl
1 Replies

6. Red Hat

Need to release Cache memory

Right now i am using Red Hat Enterprise Linux AS release 4 and cache memory occupying around 1.5GB mentioned below, total used free shared buffers cached Mem: 2026 2021 5 0 161 1477 -/+ buffers/cache: 382 1644 ... (4 Replies)
Discussion started by: thakshina
4 Replies

7. UNIX for Advanced & Expert Users

linux memory buffers & cache usage

18:45:47 # free -m total used free shared buffers cached Mem: 96679 95909 770 0 1530 19550 -/+ buffers/cache: 74828 21851 Swap: 12287 652 11635 Hi all. The below output is from a RHEL 4.5... (0 Replies)
Discussion started by: drummerrob
0 Replies

8. UNIX for Dummies Questions & Answers

High use of cache memory

Hi, I'm running a debian lenny 1GB ram, but with a high I/O. This server has 400IOPS and 3MB/s sustain. So, I noted cached memory use 800MB, buffered memory use 50MB, and no free memory is available. Questions: What does mean such a high cached memory? Who's using this cached memory? Is... (3 Replies)
Discussion started by: iga3725
3 Replies

9. Solaris

File cache portion of memory on Solaris

I'm looking to get the file cache portion of physical (real) memory on a Solaris workstation (Similar to the Cache: line in /proc/meminfo on some Linux systems): # swap -s; swap -l; vmstat 2 2; echo "::memstat" | mdb -k total: 309376k bytes allocated + 41428k reserved = 350804k used,... (5 Replies)
Discussion started by: Devyn
5 Replies

10. Windows & DOS: Issues & Discussions

Clearing of cache memory

Pls! will someone help me on how to clear my cache memory of my Internet browser Thanks Kayode (3 Replies)
Discussion started by: kayode
3 Replies

Featured Tech Videos