I know that any data could be cached from main memory to cache memory that provides faster access from the CPU than does main memory.
I know as well that each thread has it's own stack memory.
So the question is, could the program/code memory be cached as well? or is the caching exclusive to data blocks, not instructions block?
Yes, the machine instructions of the program can be cached as well. In fact, some processors have separate caches for instructions and data, while in other processors they share a single cache.
Related
I had a lot of users upload files and I find the memory not released after user uploaded files. Thus I stop the liferay tomcat, and there is no other applications, while the memory usage still high. So who cost the memory, I guess its linux server cached the documents. Can I get some idea or suggestion from you? I want to release the memory
Once Java has allocated memory from the OS, it'll not free it up again. This is not a feature of Liferay, but of the underlying JVM.
You can allocate less memory to Liferay (or the appserver) to begin with, but must be sure to at least allocate enough for the upload to be processed (AFAIK the documents aren't necessarily held in memory at the same time). You can also configure the cache sizes, so that Liferay won't need to allocate more memory from the OS, at the price of more cache misses. I'm aware of several installations that rather accepted the (minor) impact of cache misses than increasing the overall memory requirements.
However, as memory is so cheap these days, many opt to not optimize this particular aspect. If you can't upgrade your hardware it might be called for though.
Problem:
It is dedicated network topology, memory swapping (many GBs) is done via network in a kernel module. The goal is utilization (as much as possible) of network bandwidth (currently 10Gbps).
Questions:
Does Linux swapping mechanism retrieve and store a block of memory sequentially (page by page) for a particular process?
Is there a difference between memory swap in and out for a single and multi-threaded process ?
Does page-cluster applicable for this case (swap in and out)?
Does Linux swapping mechanism retrieve and store a block of memory
sequentially (page by page) for a particular process?
Regarding swapping:
I'am not sure what are you asking about, but i guess the answer is - no. It uses LRU policy to swapout inactive pages.
Regarding unswapping:
It is invoked by page fault.
Is there a difference between memory swap in and out for a single and
multi-threaded process ?
No.
Does page-cluster applicable for this case (swap in and out)?
Yes that is called readahead, when page fault is occured, usually the cluster of pages is loaded back to memory, but this could be configured.
Regarding network swap and related solutions:
NSWAP
NFS
NBD
I have a test WordPress blog on my server. Before loading a WordPress page in my broswer, my server's used memory is about 190MB. But after loading a few pages from my test blog, my server's used memory jumps to around 420MB.
I don't mind WordPress using so much memory, but what I'm struggling to understand is why does the memory never release from the system until it's rebooted?
The server doesn't seem to be slowing down but I'm concerned that it'll eventually run out of memory.
Am I paranoid or is it a genuine concern?
Without getting too technical, allocating & releasing memory are expensive operations in terms of processing resources. So long-running processes like PHP will hang onto their RAM as long as possible and re-use it, rather than asking the operating system for more memory all the time.
Apache has a setting for how many requests a particular PHP process can handle before it's killed, releasing its memory, and re-started. Chances are you aren't coming anywhere near this number of requests on your test blog.
I just found in the riak documentation that the swap makes the server unresponsive so it has to be disabled.It is also given that Riak node be allowed to be killed by the kernel if it uses too much RAM. If swap is completely disabled, Riak will simply exit. I am confused should we have to disable the swap or not?
http://docs.basho.com/riak/latest/cookbooks/Linux-Performance-Tuning/
Swap Space
Due to the heavily I/O-focused profile of Riak, swap usage
can result in the entire server becoming unresponsive. Disable swap or
otherwise implement a solution for ensuring Riak's process pages are
not swapped.
Basho recommends that the Riak node be allowed to be killed by the
kernel if it uses too much RAM. If swap is completely disabled, Riak
will simply exit when it is unable to allocate more RAM and leave a
crash dump (named erl_crash.dump) in the /var/log/riak directory which
can be used for forensics (by Basho Client Services Engineers if you
are a customer).
So no, you don't have to ... but if you don't and you use all your available RAM the machine is likely to become unresponsive.
With any (unbounded) application that performs heavy I/O where you could exhaust your system's memory that's going to be the case. Typically you would have monitoring on the machine that warned you of memory usage going past a threshold.
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Embedded system, no swap, kernel v2.6.36, memory compaction enabled.
Under heavy usage, all the RAM is tied up in cache. Cache was using about 70M of memory. When a user space process allocates memory, no problem, cache gives it up.
But there's a 3rd party device driver that seems to try to allocate a physical 5th order page, and fails with OOM. A quick look at buddyinfo confirms this... no 5th order page available. But as soon as I drop cache, plenty becomes available and the device driver no longer OOM.
So it seems to me that virtual memory allocation will trigger cache drop, but physical memory allocation will not? This doesn't make sense, because then kernel modules are likely to OOM when memory is tied up in cache, and this behavior seems to be more detrimental than slow disk access from no caching.
Is there a tuning parameter to address this?
Thanks!
So here's what's going on. I still don't know why high cache use is causing kernel modules to OOM. The problem is in 3rd party code that we don't have access to, so who knows what they're doing.
I guess one can argue if this is by design, where non-critical disk cache could take all available free memory, and cause kernel modules to OOM, then IMHO, maybe disk cache should leave something for the kernel.
I've decided to instead, limit the cache, so there is always some "truly free" memory left for kernel use, and not depend on "sort of free" memory tied up in cache.
There is a kernel patch I found that will add /proc/sys/vm/pagecache_ratio so you can set how much memory the disk cache could take. But that was never incorporated into the kernel for whatever reason (I thought that was a good idea, especially if disk cache could cause kernel OOM). But I didn't want to mess with kernel patches for maintainability and future-proofing reasons. If someone is just doing a one-shot deal, and doesn't mind patches, here's the link:
http://lwn.net/Articles/218890/
My solution is that I've recompiled the kernel and enabled cgroups, and I'm using that to limit the memory usage for a group of processes that are responsible for lots of disk access (hence running up the cache). After tweaking the configuration, it seems to be working fine. I'll leave my setup running the stress test over the weekend and see if OOM still happens.
Edit
I guess I found my own answer. There are VM tuning parameters in /proc/sys/vm/. Tune-able settings relevant to this issue are: min_free_kbytes, lowmem_reserve_ratio, and extfrag_threshold.