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Subject[NEW]: Introducing shrink_all_memory from user space
Dear All,

This is regarding a small proposal for shrink_all_memory( ) function which is found in mm/vmscan.c.
For those who are not aware, this function helps in reclaiming specified amount of physical memory and returns number of freed pages.

Currently this function is under CONFIG_HIBERNATION flag, so cannot be used by others without enabling hibernation.
Moreover this function is not exported to the outside world, so no driver can use it directly without including EXPORT_SYMBOL(shrink_all_memory) and recompiling the kernel.
The purpose of using it under hibernation(kernel/power/snapshot.c) is to regain enough physical pages to create hibernation image.

The same can be useful for some drivers who wants to allocate large contiguous memory (in MBs) but his/her system is in very bad state and could not do so without rebooting.
Due to this bad state of the system the user space application will be badly hurt on performance, and there could be a need to quickly reclaim all physical memory from user space.
This could be very helpful for small embedded products and smart phones where rebooting is never a preferred choice.

With this support in kernel, a small utility can be developed in user space which user can run and reclaim as many physical pages and noticed that his system performance is increased without rebooting.

To make this work, I have performed a sample experiment on my ubuntu(10.x) machine running with kernel-3.3.0.

Also I performed the similar experiment of one of our Samsung smart phones as well.

Following are the results from Ubuntu:

1) I downloaded kernel3.3.0 and made the respective changes in mm/vmscan.c. That is added EXPORT_SYMBOL(shrink_all_memory) under the function shrink_all_memory( ).
    (Note: CONFIG_HIBERNATION was already enabled for my system.)

2) Then I build the kernel and installed it on my Ubuntu PC.

3) After that I have developed a small kernel module (miscdevice: /dev/shrinkmem) to call shrink_all_memory( ) under my driver write( ) function.

4) Then from user space I just need to do this:

    # echo 100 > /dev/shrinkmem   (To reclaim 100MB of physical memory without reboot)

The results that were obtained with this experiment is as follows:

1) At some point of time, the buddyinfo and free pages on my Ubuntu were as follows:
root@pintu-ubuntu:~/PintuHomeTest/KERNEL_ORG# cat /proc/buddyinfo ; free -tmNode 0, zone      DMA    468     23      0      0      0      0      0      0      0      0      0
Node 0, zone   Normal    898    161     38      8      0      0      0      0      0      0      0
                 total       used       free     shared    buffers     cached
Mem:           497        489          7          0         37        405
-/+ buffers/cache:         47        449
Swap:         1458        158       1300
Total:        1956        648       1308


2) After doing "echo 100 > /dev/shrinkmem" :
[19653.833916] [SHRINKMEM]: memsize(in MB) = 100
[19653.863618] <SHRINKMEM>: Number of Pages Freed: 26756
[19653.863664] [SHRINKMEM] : Device is Closed....
Node 0, zone      DMA    411    166     51      5      0      0      0      0      0      0      0
Node 0, zone   Normal   2915   3792   2475   1335    730     23      0      0      0      0      0
                 total       used       free     shared    buffers     cached
Mem:           497        323        173          0         37        238
-/+ buffers/cache:         47        449
Swap:         1458        158       1300
Total:        1956        481       1474


3) After running again with : echo 512 > /dev/shrinkmem
[21961.064534] [SHRINKMEM]: memsize(in MB) = 512
[21961.109497] <SHRINKMEM>: Number of Pages Freed: 61078
[21961.109562] [SHRINKMEM] : Device is Closed....
Node 0, zone      DMA    116     99     87     58     16      6      1      0      0      0      0
Node 0, zone   Normal   6697   6939   5529   3756   1442    203     17      0      0      0      0
                 total       used       free     shared    buffers     cached
Mem:           497         87        410          0         37          9
-/+ buffers/cache:         40        456
Swap:         1458        158       1300
Total:        1956        245       1711

4) Thus in both the cases huge number of free pages were reclaimed.

5) After running this on my system, the performance was improved quickly.

6) I performed the same experiment on our Samsung Smart phones as well. And I have seen a drastic improve in performance after running this for 3/4 times.
    In case of phones it is more helpful as there is no swap space.

7) Your feedback and suggestion is important. Based on the feedback, I can plan to submit the patches officially after performing basic cleanups.


Future Work
==========
Our final goal is to use it during lowmem notifier where shrink_all_memory will be called automatically in background if the memory pressure falls below certain limit defined by the system.
For example we can measure the percentage memory fragmentation level of the system across each zone and if the fragmentation percentage goes above say 80-85% during lowmem notifier, we can invoke shrink_all_memory() in background.
This can be used by some application which requires large mmap or shared memory mapping.

This can be even using inside the multimedia drivers that requires large contiguous memory to check if that many memory pages can be reclaimed or not.


Please provide your valuable feedback and suggestion.


Thank you very much !


With Regards,
Pintu Kumar
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