src/devices/tech/config/low-ram.jd

page.title=Low RAM Configuration
@jd:body

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<div id="qv-wrapper">
  <div id="qv">
    <h2>In this document</h2>
    <ol id="auto-toc">
    </ol>
  </div>
</div>

<h2 id="intro">Introduction</h2>

<p>Android now supports devices with 512MB of RAM. This documentation is intended
to help OEMs optimize and configure Android 4.4 for low-memory devices. Several
of these optimizations are generic enough that they can be applied to previous
releases as well.</p>

<h2 id="optimizations">Android 4.4 platform optimizations</h2>

<h3 id="opt-mgmt">Improved memory management</h3>
<ul>
<li>Validated memory-saving kernel configurations: Kernel Same-page Merging
(KSM), and Swap to ZRAM.</li>
<li>Kill cached processes if about to be uncached and too large.</li>
<li>Don't allow large services to put themselves back into A Services (so they
can't cause the launcher to be killed).</li>
<li>Kill processes (even ordinarily unkillable ones such as the current IME)
that get too large in idle maintenance.</li>
<li>Serialize the launch of background services.</li>
<li>Tuned memory use of low-RAM devices: tighter out-of-memory (OOM) adjustment
levels, smaller graphics caches, etc.</li>
</ul>

<h3 id="opt-mem">Reduced system memory</h3>
<ul>
<li>Trimmed system_server and SystemUI processes (saved several MBs).</li>
<li>Preload dex caches in Dalvik (saved several MBs).</li>
<li>Validated JIT-off option (saves up to 1.5MB per process).</li>
<li>Reduced per-process font cache overhead.</li>
<li>Introduced ArrayMap/ArraySet and used extensively in framework as a
lighter-footprint replacement for HashMap/HashSet.</li>
</ul>

<h3 id="opt-proc">Procstats</h3>
<p>
Added a new Developer Option to show memory state and application memory usage
ranked by how often they run and amount of memory consumed.
</p>

<h3 id="opt-api">API</h3>
<p>
Added a new ActivityManager.isLowRamDevice() to allow applications to detect
when running on low memory devices and choose to disable large-RAM features.
</p>

<h3 id="opt-track">Memory tracking</h3>
<p>
New memtrack HAL to track graphics memory allocations, additional information
in dumpsys meminfo, clarified summaries in meminfo (for example reported free
RAM includes RAM of cached processes, so that OEMs don't try to optimize the
wrong thing).
</p>

<h2 id="build-time">Build-time configuration</h2>
<h3 id="flag">Enable Low Ram Device flag</h3>
<p>We are introducing a new API called
<code>ActivityManager.isLowRamDevice()</code> for applications to  determine if
they should turn off specific memory-intensive
  features that work poorly on low-memory devices.</p>
<p>For 512MB devices, this API is expected to return: "true" It can be enabled by
  the following system property in the device makefile.<br/>
<code>PRODUCT_PROPERTY_OVERRIDES += ro.config.low_ram=true</code></p>

<h3 id="jit">Disable JIT</h3>

  <p>System-wide JIT memory usage is dependent on the number of applications
  running and the code footprint of those applications. The JIT establishes a
  maximum translated code cache size and touches the pages within it as needed.
  JIT costs somewhere between 3M and 6M across a typical running system.<br/>
  <br/>
  The large apps tend to max out the code cache fairly quickly (which by default
  has been 1M). On average, JIT cache usage runs somewhere between 100K and 200K
  bytes per app. Reducing the max size of the cache can help somewhat with
  memory usage, but if set too low will send the JIT into a thrashing mode.  For
the really low-memory devices, we recommend the JIT be disabled entirely.</p>

<p>This can be achieved by adding the following line to the product makefile:<br/>
<code>PRODUCT_PROPERTY_OVERRIDES += dalvik.vm.jit.codecachesize=0</code></p>
<h3 id="launcher">Launcher Configs</h3>


  <p>Ensure the default wallpaper setup on launcher is <strong>not</strong>
using live-wallpaper. Low-memory devices should not pre-install any live wallpapers. </p>


<h2 id="kernel">Kernel configuration</h2>
<h3 id="kernel-tuning">Tuning kernel/ActivityManager to reduce direct reclaim </h3>


  <p>Direct reclaim happens when a process or the kernel tries to allocate a page
  of memory (either directly or due to faulting in a new page) and the kernel
  has used all available free memory. This requires the kernel to block the
  allocation while it frees up a page. This in turn often requires disk I/O to
  flush out a dirty file-backed page or waiting for <code>lowmemorykiller</code> to kill a
  process. This can result in extra I/O in any thread, including a UI thread.</p>

  <p>To avoid direct reclaim, the kernel has watermarks that trigger <code>kswapd</code> or
  background reclaim.  This is a thread that tries to free up pages so the next
  time a real thread allocates it can succeed quickly.</p>

  <p>The default threshold to trigger background reclaim is fairly low, around 2MB
  on a 2GB device and 636KB on a 512MB device. And the kernel reclaims only a
  few MB of memory in background reclaim. This means any process that quickly
  allocates more than a few megabytes is going to quickly hit direct reclaim.</p>

<p>Support for a new kernel tunable is added in the android-3.4 kernel branch as
  patch 92189d47f66c67e5fd92eafaa287e153197a454f ("add extra free kbytes
  tunable").  Cherry-picking this patch to a device's kernel will allow
  ActivityManager to tell the kernel to try to keep 3 full-screen 32 bpp buffers
  of memory free.</p>

<p>These thresholds can be configured via the framework config.xml</p>

<pre>
&lt;!-- Device configuration setting the /proc/sys/vm/extra_free_kbytes tunable
in the kernel (if it exists).  A high value will increase the amount of memory
that the kernel tries to keep free, reducing allocation time and causing the
lowmemorykiller to kill earlier.  A low value allows more memory to be used by
processes but may cause more allocations to block waiting on disk I/O or
lowmemorykiller.  Overrides the default value chosen by ActivityManager based
on screen size.  0 prevents keeping any extra memory over what the kernel keeps
by default.  -1 keeps the default. --&gt;
&lt;integer name=&quot;config_extraFreeKbytesAbsolute&quot;&gt;-1&lt;/integer&gt;
</pre>

<pre>
&lt;!-- Device configuration adjusting the /proc/sys/vm/extra_free_kbytes
tunable in the kernel (if it exists).  0 uses the default value chosen by
ActivityManager.  A positive value  will increase the amount of memory that the
kernel tries to keep free, reducing allocation time and causing the
lowmemorykiller to kill earlier.  A negative value allows more memory to be
used by processes but may cause more allocations to block waiting on disk I/O
or lowmemorykiller.  Directly added to the default value chosen by
ActivityManager based on screen size. --&gt;
&lt;integer name=&quot;config_extraFreeKbytesAdjust&quot;&gt;0&lt;/integer&gt;
</pre>

<h3 id="lowmem">Tuning LowMemoryKiller</h3>

<p>ActivityManager configures the thresholds of the LowMemoryKiller to match its
expectation of the working set of file-backed pages (cached pages) required to
run the processes in each priority level bucket.  If a device has high
requirements for the working set, for example if the vendor UI requires more
memory or if more services have been added, the thresholds can be increased. </p>

<p>The thresholds can be reduced if too much memory is being reserved for file
backed pages, so that background processes are being killed long before disk
thrashing would occur due to the cache getting too small.</p>

<pre>
&lt;!-- Device configuration setting the minfree tunable in the lowmemorykiller
in the kernel.  A high value will cause the lowmemorykiller to fire earlier,
keeping more memory in the file cache and preventing I/O thrashing, but
allowing fewer processes to stay in memory.  A low value will keep more
processes in memory but may cause thrashing if set too low.  Overrides the
default value chosen by ActivityManager based on screen size and total memory
for the largest lowmemorykiller bucket, and scaled proportionally to the
smaller buckets.  -1 keeps the default. --&gt;
&lt;integer name=&quot;config_lowMemoryKillerMinFreeKbytesAbsolute&quot;&gt;-1&lt;/integer&gt;
</pre>

<pre>
&lt;!-- Device configuration adjusting the minfree tunable in the
lowmemorykiller in the kernel.  A high value will cause the lowmemorykiller to
fire earlier, keeping more memory in the file cache and preventing I/O
thrashing, but allowing fewer processes to stay in memory.  A low value will
keep more processes in memory but may cause thrashing if set too low.  Directly
added to the default value chosen by          ActivityManager based on screen
size and total memory for the largest lowmemorykiller bucket, and scaled
proportionally to the smaller buckets. 0 keeps the default. --&gt;
&lt;integer name=&quot;config_lowMemoryKillerMinFreeKbytesAdjust&quot;&gt;0&lt;/integer&gt;
</pre>

<h3 id="ksm">KSM (Kernel samepage merging)</h3>

<p>KSM is a kernel thread that runs in the background and compares pages in
memory that have been marked <code>MADV_MERGEABLE</code> by user-space. If two pages are
found to be the same, the KSM thread merges them back as a single
copy-on-write page of memory.</p>

<p>KSM will save memory over time on a running system, gaining memory duplication
at a cost of CPU power, which could have an impact on battery life. You should
measure whether the power tradeoff is worth the memory savings you get by
enabling KSM.</p>

<p>To test KSM, we recommend looking at long running devices (several hours) and
seeing whether KSM makes any noticeable improvement on launch times and
rendering times.</p>

<p>To enable KSM, enable <code>CONFIG_KSM</code> in the kernel and then add the
following lines to your` <code>init.&lt;device&gt;.rc</code> file:<br>

<pre>
write /sys/kernel/mm/ksm/pages_to_scan 100
write /sys/kernel/mm/ksm/sleep_millisecs 500
write /sys/kernel/mm/ksm/run 1
</pre>

<p>Once enabled, there are few utilities that will help in the debugging namely :
procrank, librank, &amp; ksminfo. These utilities allow you to see which KSM
memory is mapped to what process, which processes use the most KSM memory.
Once you have found a chunk of memory that looks worth exploring you can use
either the hat utility if it's a duplicate object on the dalvik heap. </p>

<h3 id="zram">Swap to zRAM</h3>

<p>zRAM swap can increase the amount of memory available in the system by
compressing memory pages and putting them in a dynamically allocated swap area
of memory.</p>

<p>Again, since this is trading off CPU time for a small increase in memory, you
should be careful about measuring the performance impact zRAM swap has on your
system.</p>

<p>Android handles swap to zRAM at several levels:</p>

<ul>
  <li>First, the following kernel options must be enabled to use zRAM swap
    effectively:
    <ul>
      <li><code>CONFIG_SWAP</code></li>
      <li><code>CONFIG_CGROUP_MEM_RES_CTLR</code></li>
      <li><code>CONFIG_CGROUP_MEM_RES_CTLR_SWAP</code></li>
      <li><code>CONFIG_ZRAM</code></li>
    </ul>
  </li>
  <li>Then, you should add a line that looks like this to your fstab:<br />
    <code>/dev/block/zram0 none swap defaults zramsize=&lt;size in bytes&gt;,swapprio=&lt;swap partition priority&gt;</code><br />
  <code><br />
  zramsize</code> is mandatory and indicates how much uncompressed memory you want
    the zram area to hold. Compression ratios in the 30-50% range are usually
  observed.<br />
  <br />
  <code>swapprio</code> is optional and not needed if you don't have more than one swap
  area.<br />
  <br />
  You should also be sure to label the associated block device as a swap_block_device
  in the device-specific <a href="{@docRoot}security/selinux/implement.html">
  sepolicy/file_contexts</a> so that it is treated properly by SELinux. <br />
  <code>/dev/block/zram0 u:object_r:swap_block_device:s0</code><br />
  <br />
  </li>
  <li>By default, the Linux kernel swaps in 8 pages of memory at a time. When
    using ZRAM, the incremental cost of reading 1 page at a time is negligible
    and may help in case the device is under extreme memory pressure. To read
    only 1 page at a time, add the following to your <code>init.rc</code>:<br />
  <code>write /proc/sys/vm/page-cluster 0</code></li>
  <li>In your <code>init.rc</code> after the <code>mount_all /fstab.X</code> line, add:<br />
  <code>swapon_all /fstab.X</code></li>
  <li>The memory cgroups are automatically configured at boot time if the
    feature is enabled in kernel.</li>
  <li>If memory cgroups are available, the ActivityManager will mark lower
    priority threads as being more swappable than other threads. If memory is
    needed, the Android kernel will start migrating memory pages to zRAM swap,
    giving a higher priority to those memory pages that have been marked by
    ActivityManager. </li>
</ul>

<h3 id="carveouts">Carveouts, Ion and Contiguous Memory Allocation (CMA)</h3>

<p>It is especially important on low memory devices to be mindful about
carveouts, especially those that will not always be fully utilized -- for
example a carveout for secure video playback. There are several solutions to
minimizing the impact of your carveout regions that depend on the exact
requirements of your hardware.</p>

<p>If hardware permits discontiguous memory allocations, the ion system heap
allows memory allocations from system memory,
eliminating the need for a carveout. It also attempts to make large
allocations to eliminate TLB pressure on peripherals. If memory regions must
be contiguous or confined to a specific address range, the contiguous memory
allocator (CMA) can be used.</p>

<p>This creates a carveout that the system can also use of for movable pages.
When the region is needed, movable pages will be migrated out of it, allowing
the system to use a large carveout for other purposes when it is free. CMA can
be used directly or more simply via ion by using the ion cma heap.</p>

<h2 id="app-opts">Application optimization tips</h2>
<ul>
   <li>Review <a
href="http://developer.android.com/training/articles/memory.html">Managing your
App's Memory</a> and these past blog posts on the same topic:
  <ul>
    <li><a
href="http://android-developers.blogspot.com/2009/01/avoiding-memory-leaks.html">http://android-developers.blogspot.com/2009/01/avoiding-memory-leaks.html</a></li>
    <li><a
href="http://android-developers.blogspot.com/2011/03/memory-analysis-for-android.html">http://android-developers.blogspot.com/2011/03/memory-analysis-for-android.html</a></li>
    <li><a
href="http://android-developers.blogspot.com/2009/02/track-memory-allocations.html">http://android-developers.blogspot.com/2009/02/track-memory-allocations.html</a></li>
    <li> <a
href="http://tools.android.com/recent/lintperformancechecks">http://tools.android.com/recent/lintperformancechecks</a></li>
    </ul>
</li>
  <li>Check/remove any unused assets from preinstalled apps -
development/tools/findunused (should help make the app smaller).</li>
<li>Use PNG format for assets, especially when they have transparent areas</li>
<li>If writing native code, use calloc() rather than malloc/memset</li>
<li>Don't enable code that is writing Parcel data to disk and reading it later.</li>
<li>Don't subscribe to every package installed, instead use ssp filtering. Add
filtering like below:
<br />
  <code>&lt;data android:scheme=&quot;package&quot; android:ssp=&quot;com.android.pkg1&quot; /&gt;<br />
  &lt;data android:scheme=&quot;package&quot; android:ssp=&quot;com.myapp.act1&quot; /&gt;</code></li>
</ul>

<h3 id="process-states">Understand the various process states in Android</h3>

  <ul>
  <li><p>SERVICE - SERVICE_RESTARTING<br/>
  Applications that are making themselves run in the background for their own
  reason.  Most common problem apps have when they run in the background too
  much.  %duration * pss is probably a good "badness" metric, although this set
  is so focused that just doing %duration is probably better to focus on the
  fact that we just don't want them running at all.</p></li>
  <li><p>IMPORTANT_FOREGROUND - RECEIVER<br/>
  Applications running in the background (not directly interacting with the
  user) for any reason.  These all add memory load to the system.  In this case
  the (%duration * pss) badness value is probably the best ordering of such
  processes, because many of these will be always running for good reason, and
  their pss size then is very important as part of their memory load.</p></li>
  <li><p>PERSISTENT<br/>
  Persistent system processes.  Track pss to watch for these processes getting
  too large.</p></li>
  <li><p>TOP<br/>
  Process the user is currently interacting with.  Again, pss is the important
  metric here, showing how much memory load the app is creating while in use.</p></li>
  <li><p>HOME - CACHED_EMPTY<br/>
  All of these processes at the bottom are ones that the system is keeping
  around in case they are needed again; but they can be freely killed at any
  time and re-created if needed.  These are the basis for how we compute the
  memory state -- normal, moderate, low, critical is based on how many of these
  processes the system can keep around.  Again the key thing for these processes
  is the pss; these processes should try to get their memory footprint down as
  much as possible when they are in this state, to allow for the maximum total
  number of processes to be kept around.  Generally a well behaved app will have
  a pss footprint that is significantly smaller when in this state than when
  TOP.</p></li>
  <li>
    <p>TOP vs. CACHED_ACTIVITY-CACHED_ACTIVITY_CLIENT<em><br/>
  </em>The difference in pss between when a process is TOP vs. when it is in either
  of these specific cached states is the best data for seeing how well it is
  releasing memory when going into the background.  Excluding CACHED_EMPTY state
  makes this data better, since it removes situations when the process has
  started for some reasons besides doing UI and so will not have to deal with
  all of the UI overhead it gets when interacting with the user.</p></li>
  </ul>

<h2 id="analysis">Analysis</h2>

<h3 id="app-startup">Analyzing app startup time</h3>

<p>Use <code>$ adb shell am start</code> with the <code>-P</code> or
<code>--start-profiler</code> option to run the profiler when your app starts.
This will start the profiler almost immediately after your process is forked
from zygote, before any of your code is loaded into it.</p>

<h3 id="bug-reports">Analyze using bugreports </h3>

<p>Now contains various information that can be used for debugging. The
services include <code>batterystats</code>, <code>netstats</code>,
<code>procstats</code>, and <code>usagestats</code>. You can find them with
lines like this:</p>

<pre>
------ CHECKIN BATTERYSTATS (dumpsys batterystats --checkin) ------
7,0,h,-2558644,97,1946288161,3,2,0,340,4183
7,0,h,-2553041,97,1946288161,3,2,0,340,4183
</pre>

<h3 id="persistent">Check for any persistent processes</h3>

<p>Reboot the device and check the processes.<br/>
Run for a few hours and check the processes again. There should not be any
long running processes.</p>

<h3 id="longevity">Run longevity tests</h3>

<p>Run for longer durations and track the memory of the process. Does it
increase? Does it stay constant? Create Canonical use cases and run longevity
tests on these scenarios</p>