279 lines
15 KiB
Plaintext
Executable File
279 lines
15 KiB
Plaintext
Executable File
Frontswap provides a "transcendent memory" interface for swap pages.
|
|
In some environments, dramatic performance savings may be obtained because
|
|
swapped pages are saved in RAM (or a RAM-like device) instead of a swap disk.
|
|
|
|
(Note, frontswap -- and cleancache (merged at 3.0) -- are the "frontends"
|
|
and the only necessary changes to the core kernel for transcendent memory;
|
|
all other supporting code -- the "backends" -- is implemented as drivers.
|
|
See the LWN.net article "Transcendent memory in a nutshell" for a detailed
|
|
overview of frontswap and related kernel parts:
|
|
https://lwn.net/Articles/454795/ )
|
|
|
|
Frontswap is so named because it can be thought of as the opposite of
|
|
a "backing" store for a swap device. The storage is assumed to be
|
|
a synchronous concurrency-safe page-oriented "pseudo-RAM device" conforming
|
|
to the requirements of transcendent memory (such as Xen's "tmem", or
|
|
in-kernel compressed memory, aka "zcache", or future RAM-like devices);
|
|
this pseudo-RAM device is not directly accessible or addressable by the
|
|
kernel and is of unknown and possibly time-varying size. The driver
|
|
links itself to frontswap by calling frontswap_register_ops to set the
|
|
frontswap_ops funcs appropriately and the functions it provides must
|
|
conform to certain policies as follows:
|
|
|
|
An "init" prepares the device to receive frontswap pages associated
|
|
with the specified swap device number (aka "type"). A "store" will
|
|
copy the page to transcendent memory and associate it with the type and
|
|
offset associated with the page. A "load" will copy the page, if found,
|
|
from transcendent memory into kernel memory, but will NOT remove the page
|
|
from transcendent memory. An "invalidate_page" will remove the page
|
|
from transcendent memory and an "invalidate_area" will remove ALL pages
|
|
associated with the swap type (e.g., like swapoff) and notify the "device"
|
|
to refuse further stores with that swap type.
|
|
|
|
Once a page is successfully stored, a matching load on the page will normally
|
|
succeed. So when the kernel finds itself in a situation where it needs
|
|
to swap out a page, it first attempts to use frontswap. If the store returns
|
|
success, the data has been successfully saved to transcendent memory and
|
|
a disk write and, if the data is later read back, a disk read are avoided.
|
|
If a store returns failure, transcendent memory has rejected the data, and the
|
|
page can be written to swap as usual.
|
|
|
|
If a backend chooses, frontswap can be configured as a "writethrough
|
|
cache" by calling frontswap_writethrough(). In this mode, the reduction
|
|
in swap device writes is lost (and also a non-trivial performance advantage)
|
|
in order to allow the backend to arbitrarily "reclaim" space used to
|
|
store frontswap pages to more completely manage its memory usage.
|
|
|
|
Note that if a page is stored and the page already exists in transcendent memory
|
|
(a "duplicate" store), either the store succeeds and the data is overwritten,
|
|
or the store fails AND the page is invalidated. This ensures stale data may
|
|
never be obtained from frontswap.
|
|
|
|
If properly configured, monitoring of frontswap is done via debugfs in
|
|
the /sys/kernel/debug/frontswap directory. The effectiveness of
|
|
frontswap can be measured (across all swap devices) with:
|
|
|
|
failed_stores - how many store attempts have failed
|
|
loads - how many loads were attempted (all should succeed)
|
|
succ_stores - how many store attempts have succeeded
|
|
invalidates - how many invalidates were attempted
|
|
|
|
A backend implementation may provide additional metrics.
|
|
|
|
FAQ
|
|
|
|
1) Where's the value?
|
|
|
|
When a workload starts swapping, performance falls through the floor.
|
|
Frontswap significantly increases performance in many such workloads by
|
|
providing a clean, dynamic interface to read and write swap pages to
|
|
"transcendent memory" that is otherwise not directly addressable to the kernel.
|
|
This interface is ideal when data is transformed to a different form
|
|
and size (such as with compression) or secretly moved (as might be
|
|
useful for write-balancing for some RAM-like devices). Swap pages (and
|
|
evicted page-cache pages) are a great use for this kind of slower-than-RAM-
|
|
but-much-faster-than-disk "pseudo-RAM device" and the frontswap (and
|
|
cleancache) interface to transcendent memory provides a nice way to read
|
|
and write -- and indirectly "name" -- the pages.
|
|
|
|
Frontswap -- and cleancache -- with a fairly small impact on the kernel,
|
|
provides a huge amount of flexibility for more dynamic, flexible RAM
|
|
utilization in various system configurations:
|
|
|
|
In the single kernel case, aka "zcache", pages are compressed and
|
|
stored in local memory, thus increasing the total anonymous pages
|
|
that can be safely kept in RAM. Zcache essentially trades off CPU
|
|
cycles used in compression/decompression for better memory utilization.
|
|
Benchmarks have shown little or no impact when memory pressure is
|
|
low while providing a significant performance improvement (25%+)
|
|
on some workloads under high memory pressure.
|
|
|
|
"RAMster" builds on zcache by adding "peer-to-peer" transcendent memory
|
|
support for clustered systems. Frontswap pages are locally compressed
|
|
as in zcache, but then "remotified" to another system's RAM. This
|
|
allows RAM to be dynamically load-balanced back-and-forth as needed,
|
|
i.e. when system A is overcommitted, it can swap to system B, and
|
|
vice versa. RAMster can also be configured as a memory server so
|
|
many servers in a cluster can swap, dynamically as needed, to a single
|
|
server configured with a large amount of RAM... without pre-configuring
|
|
how much of the RAM is available for each of the clients!
|
|
|
|
In the virtual case, the whole point of virtualization is to statistically
|
|
multiplex physical resources across the varying demands of multiple
|
|
virtual machines. This is really hard to do with RAM and efforts to do
|
|
it well with no kernel changes have essentially failed (except in some
|
|
well-publicized special-case workloads).
|
|
Specifically, the Xen Transcendent Memory backend allows otherwise
|
|
"fallow" hypervisor-owned RAM to not only be "time-shared" between multiple
|
|
virtual machines, but the pages can be compressed and deduplicated to
|
|
optimize RAM utilization. And when guest OS's are induced to surrender
|
|
underutilized RAM (e.g. with "selfballooning"), sudden unexpected
|
|
memory pressure may result in swapping; frontswap allows those pages
|
|
to be swapped to and from hypervisor RAM (if overall host system memory
|
|
conditions allow), thus mitigating the potentially awful performance impact
|
|
of unplanned swapping.
|
|
|
|
A KVM implementation is underway and has been RFC'ed to lkml. And,
|
|
using frontswap, investigation is also underway on the use of NVM as
|
|
a memory extension technology.
|
|
|
|
2) Sure there may be performance advantages in some situations, but
|
|
what's the space/time overhead of frontswap?
|
|
|
|
If CONFIG_FRONTSWAP is disabled, every frontswap hook compiles into
|
|
nothingness and the only overhead is a few extra bytes per swapon'ed
|
|
swap device. If CONFIG_FRONTSWAP is enabled but no frontswap "backend"
|
|
registers, there is one extra global variable compared to zero for
|
|
every swap page read or written. If CONFIG_FRONTSWAP is enabled
|
|
AND a frontswap backend registers AND the backend fails every "store"
|
|
request (i.e. provides no memory despite claiming it might),
|
|
CPU overhead is still negligible -- and since every frontswap fail
|
|
precedes a swap page write-to-disk, the system is highly likely
|
|
to be I/O bound and using a small fraction of a percent of a CPU
|
|
will be irrelevant anyway.
|
|
|
|
As for space, if CONFIG_FRONTSWAP is enabled AND a frontswap backend
|
|
registers, one bit is allocated for every swap page for every swap
|
|
device that is swapon'd. This is added to the EIGHT bits (which
|
|
was sixteen until about 2.6.34) that the kernel already allocates
|
|
for every swap page for every swap device that is swapon'd. (Hugh
|
|
Dickins has observed that frontswap could probably steal one of
|
|
the existing eight bits, but let's worry about that minor optimization
|
|
later.) For very large swap disks (which are rare) on a standard
|
|
4K pagesize, this is 1MB per 32GB swap.
|
|
|
|
When swap pages are stored in transcendent memory instead of written
|
|
out to disk, there is a side effect that this may create more memory
|
|
pressure that can potentially outweigh the other advantages. A
|
|
backend, such as zcache, must implement policies to carefully (but
|
|
dynamically) manage memory limits to ensure this doesn't happen.
|
|
|
|
3) OK, how about a quick overview of what this frontswap patch does
|
|
in terms that a kernel hacker can grok?
|
|
|
|
Let's assume that a frontswap "backend" has registered during
|
|
kernel initialization; this registration indicates that this
|
|
frontswap backend has access to some "memory" that is not directly
|
|
accessible by the kernel. Exactly how much memory it provides is
|
|
entirely dynamic and random.
|
|
|
|
Whenever a swap-device is swapon'd frontswap_init() is called,
|
|
passing the swap device number (aka "type") as a parameter.
|
|
This notifies frontswap to expect attempts to "store" swap pages
|
|
associated with that number.
|
|
|
|
Whenever the swap subsystem is readying a page to write to a swap
|
|
device (c.f swap_writepage()), frontswap_store is called. Frontswap
|
|
consults with the frontswap backend and if the backend says it does NOT
|
|
have room, frontswap_store returns -1 and the kernel swaps the page
|
|
to the swap device as normal. Note that the response from the frontswap
|
|
backend is unpredictable to the kernel; it may choose to never accept a
|
|
page, it could accept every ninth page, or it might accept every
|
|
page. But if the backend does accept a page, the data from the page
|
|
has already been copied and associated with the type and offset,
|
|
and the backend guarantees the persistence of the data. In this case,
|
|
frontswap sets a bit in the "frontswap_map" for the swap device
|
|
corresponding to the page offset on the swap device to which it would
|
|
otherwise have written the data.
|
|
|
|
When the swap subsystem needs to swap-in a page (swap_readpage()),
|
|
it first calls frontswap_load() which checks the frontswap_map to
|
|
see if the page was earlier accepted by the frontswap backend. If
|
|
it was, the page of data is filled from the frontswap backend and
|
|
the swap-in is complete. If not, the normal swap-in code is
|
|
executed to obtain the page of data from the real swap device.
|
|
|
|
So every time the frontswap backend accepts a page, a swap device read
|
|
and (potentially) a swap device write are replaced by a "frontswap backend
|
|
store" and (possibly) a "frontswap backend loads", which are presumably much
|
|
faster.
|
|
|
|
4) Can't frontswap be configured as a "special" swap device that is
|
|
just higher priority than any real swap device (e.g. like zswap,
|
|
or maybe swap-over-nbd/NFS)?
|
|
|
|
No. First, the existing swap subsystem doesn't allow for any kind of
|
|
swap hierarchy. Perhaps it could be rewritten to accommodate a hierarchy,
|
|
but this would require fairly drastic changes. Even if it were
|
|
rewritten, the existing swap subsystem uses the block I/O layer which
|
|
assumes a swap device is fixed size and any page in it is linearly
|
|
addressable. Frontswap barely touches the existing swap subsystem,
|
|
and works around the constraints of the block I/O subsystem to provide
|
|
a great deal of flexibility and dynamicity.
|
|
|
|
For example, the acceptance of any swap page by the frontswap backend is
|
|
entirely unpredictable. This is critical to the definition of frontswap
|
|
backends because it grants completely dynamic discretion to the
|
|
backend. In zcache, one cannot know a priori how compressible a page is.
|
|
"Poorly" compressible pages can be rejected, and "poorly" can itself be
|
|
defined dynamically depending on current memory constraints.
|
|
|
|
Further, frontswap is entirely synchronous whereas a real swap
|
|
device is, by definition, asynchronous and uses block I/O. The
|
|
block I/O layer is not only unnecessary, but may perform "optimizations"
|
|
that are inappropriate for a RAM-oriented device including delaying
|
|
the write of some pages for a significant amount of time. Synchrony is
|
|
required to ensure the dynamicity of the backend and to avoid thorny race
|
|
conditions that would unnecessarily and greatly complicate frontswap
|
|
and/or the block I/O subsystem. That said, only the initial "store"
|
|
and "load" operations need be synchronous. A separate asynchronous thread
|
|
is free to manipulate the pages stored by frontswap. For example,
|
|
the "remotification" thread in RAMster uses standard asynchronous
|
|
kernel sockets to move compressed frontswap pages to a remote machine.
|
|
Similarly, a KVM guest-side implementation could do in-guest compression
|
|
and use "batched" hypercalls.
|
|
|
|
In a virtualized environment, the dynamicity allows the hypervisor
|
|
(or host OS) to do "intelligent overcommit". For example, it can
|
|
choose to accept pages only until host-swapping might be imminent,
|
|
then force guests to do their own swapping.
|
|
|
|
There is a downside to the transcendent memory specifications for
|
|
frontswap: Since any "store" might fail, there must always be a real
|
|
slot on a real swap device to swap the page. Thus frontswap must be
|
|
implemented as a "shadow" to every swapon'd device with the potential
|
|
capability of holding every page that the swap device might have held
|
|
and the possibility that it might hold no pages at all. This means
|
|
that frontswap cannot contain more pages than the total of swapon'd
|
|
swap devices. For example, if NO swap device is configured on some
|
|
installation, frontswap is useless. Swapless portable devices
|
|
can still use frontswap but a backend for such devices must configure
|
|
some kind of "ghost" swap device and ensure that it is never used.
|
|
|
|
5) Why this weird definition about "duplicate stores"? If a page
|
|
has been previously successfully stored, can't it always be
|
|
successfully overwritten?
|
|
|
|
Nearly always it can, but no, sometimes it cannot. Consider an example
|
|
where data is compressed and the original 4K page has been compressed
|
|
to 1K. Now an attempt is made to overwrite the page with data that
|
|
is non-compressible and so would take the entire 4K. But the backend
|
|
has no more space. In this case, the store must be rejected. Whenever
|
|
frontswap rejects a store that would overwrite, it also must invalidate
|
|
the old data and ensure that it is no longer accessible. Since the
|
|
swap subsystem then writes the new data to the read swap device,
|
|
this is the correct course of action to ensure coherency.
|
|
|
|
6) What is frontswap_shrink for?
|
|
|
|
When the (non-frontswap) swap subsystem swaps out a page to a real
|
|
swap device, that page is only taking up low-value pre-allocated disk
|
|
space. But if frontswap has placed a page in transcendent memory, that
|
|
page may be taking up valuable real estate. The frontswap_shrink
|
|
routine allows code outside of the swap subsystem to force pages out
|
|
of the memory managed by frontswap and back into kernel-addressable memory.
|
|
For example, in RAMster, a "suction driver" thread will attempt
|
|
to "repatriate" pages sent to a remote machine back to the local machine;
|
|
this is driven using the frontswap_shrink mechanism when memory pressure
|
|
subsides.
|
|
|
|
7) Why does the frontswap patch create the new include file swapfile.h?
|
|
|
|
The frontswap code depends on some swap-subsystem-internal data
|
|
structures that have, over the years, moved back and forth between
|
|
static and global. This seemed a reasonable compromise: Define
|
|
them as global but declare them in a new include file that isn't
|
|
included by the large number of source files that include swap.h.
|
|
|
|
Dan Magenheimer, last updated April 9, 2012
|