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Is there a way to detect memory fragmentation on linux ? This is because on some long running servers I have noticed performance degradation and only after I restart process I see better performance. I noticed it more when using linux huge page support -- are huge pages in linux more prone to fragmentation ?

I have looked at /proc/buddyinfo in particular. I want to know whether there are any better ways(not just CLI commands per se, any program or theoretical background would do) to look at it.

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I am not looking at just quick commandline solutions, any simple program/theory will also do. Hence, I did not ask at serverfault. –  Raghu Apr 16 '10 at 9:37
I don't understand here one point. As far as I understand memory fragmentation must lead to lack of memory and as a result to memory allocation errors. However you are asking about performance degradation. Is it because you have lots of memory swapped to disk? And if so what give vmstat in the field so? –  skwllsp Apr 16 '10 at 10:49
@skwllsp - Edited my answer to be more specific. –  Tim Post Apr 16 '10 at 12:48
@Raghu - I would not expect most system administrators to modify kernel code to make memory management behave differently, however, skilled Linux admins should know at least an overview of how Linux manages memory. This question is really on the line. I voted to migrate it simply because I can't suggest (in my answer) code that answers your question. Reading from /proc or using vmstat is a common user experience. If you were writing a program to do the same, it would be different. If you intend to use bash to harvest this info, edit your question, it won't be closed :) –  Tim Post Apr 16 '10 at 13:47
@Tim - As i suggested it is not just the bash/cli commands I wanted to know, I needed the information to help me in my benchmarking procedure(to analyze the results, not to run them) . –  Raghu Apr 16 '10 at 14:11
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migrated from stackoverflow.com Apr 17 '10 at 4:40

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2 Answers

I am answering to the tag. My answer is specific only to Linux.

Yes, huge pages are more prone to fragmentation. There are two views of memory, the one your process gets (virtual) and the one the kernel manages (real). The larger any page, the more difficult it's going to be to group (and keep it with) its neighbors, especially when your service is running on a system that also has to support others that by default allocate and write to way more memory than they actually end up using.

The kernel's mapping of (real) granted addresses is private. There's a very good reason why userspace sees them as the kernel presents them, because the kernel needs to be able to overcommit without confusing userspace. Your process gets a nice, contiguous "Disneyfied" address space in which to work, oblivious of what the kernel is actually doing with that memory behind the scenes.

The reason you see degraded performance on long running servers is most likely because allocated blocks that have not been explicitly locked (e.g. mlock()/mlockall() or posix_madvise()) and not modified in a while have been paged out, which means your service skids to disk when it has to read them. Modifying this behavior makes your process a bad neighbor, which is why many people put their RDBMS on a completely different server than web/php/python/ruby/whatever. The only way to fix that, sanely, is to reduce the competition for contiguous blocks.

Fragmentation is only really noticeable (in most cases) when page A is in memory and page B has moved to swap. Naturally, re-starting your service would seem to 'cure' this, but only because the kernel has not yet had an opportunity to page out the process' (now) newly allocated blocks within the confines of its overcommit ratio.

In fact, re-starting (lets say) 'apache' under a high load is likely going to send blocks owned by other services straight to disk. So yes, 'apache' would improve for a short time, but 'mysql' might suffer .. at least until the kernel makes them suffer equally when there is simply a lack of ample physical memory.

Add more memory, or split up demanding malloc() consumers :) Its not just fragmentation that you need to be looking at.

Try vmstat to get an overview of what's actually being stored where.

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Thank you for the answer. I was using huge pages(size = 2048KB each) for mysql - innodb buffer pool - to see how well it fares(using sysbench). Initially when the process uptime (and even system uptime) was low, it was giving very good results. However, its performance started to degrade over several runs. Regarding the page out you mentioned, I surely noticed a high VM activity, but I presumed it may have been because of benchmark and innodb log flushing(vm activity higher with huge pages than without). I also set vm.swappiness to 1. I could not notice any drastic change. –  Raghu Apr 16 '10 at 13:49
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Using huge pages should not cause extra memory fragmentation on Linux; Linux support for huge pages is only for shared memory (via shmget or mmap), and any huge pages used must be specifically requested and preallocated by a system admin. Once in memory, they are pinned there, and are not swapped out. The challenge of swapping in huge pages in the face of memory fragmentation is exactly why they remain pinned in memory (when allocating a 2MB huge page, the kernel must find 512 contiguous free 4KB pages, which may not even exist).

Linux documentation on huge pages: http://lwn.net/Articles/375098/

There is one circumstance where memory fragmentation could cause huge page allocation to be slow (but not where huge pages cause memory fragmentation), and that's if your system is configured to grow the pool of huge pages if requested by an application. If /proc/sys/vm/nr_overcommit_hugepages is greater than /proc/sys/vm/nr_hugepages, this might happen.

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Indeed - and it should generally help performance because it will prevent TLB misses (see the linked article for explanation). –  Dan Pritts Dec 16 '13 at 15:21
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