Xen devel - Oct 2008 - [RFC] Replacing Xen''s xmalloc engine and(?) API

Xen hypervisor experts --

I''m looking at replacing the engine of Xen''s xmalloc/xfree
(dynamic memory allocation) with a low fragmentation, fixed
overhead dynamic allocator called TLSF ("two-level sequential
fit").  TLSF info can be found here:

http://rtportal.upv.es/rtmalloc/

TLSF has been modified to work with Linux and for an unlimited
collection of page-sized regions by Nitin Gupta here:

http://code.google.com/p/compcache/wiki/TLSFAllocator

The code I started with (thanks Nitin!) is here:

http://code.google.com/p/compcache/source/browse/trunk/sub-projects/allocators/tlsf-kmod

(Note that a lot of this code can be stripped out for Xen.)

Xen''s current xmalloc engine (essentially a first-fit algorithm) is
adequate for current needs, but I''m working on a project (see
"Paravirtualized Paging" at WIOV in December) that emphasizes its
weaknesses, the largest being that maximum calltime is essentially
unbounded, especially when allocating a large number of near-page-sized
items.  (I''m measuring millions of cycles to do some xfree''s.)
Another
project that may soon make it into Xen (see "Difference Engine" at
OSDI in December) has similarly intense near-page-sized needs.

Anyway, I''ve hacked a version of TLSF into Xen, enough to meet
my needs for now, but one of the xmalloc weaknesses (from my pov
anyway) is built into the API: xmalloc''ing a page actually
allocates two contiguous pages, xmalloc''ing two pages actually
allocates four and so on.  Worse, xmalloc''ing slightly LESS than
a page, actually allocates two contiguous pages, etc., because
the current mechanism requires space for a block header.

As a result, I''d like to propose a change to the xmalloc interface
to make this issue more explicit:  I''d like to change xmalloc/xfree
to FAIL on allocation sizes greater than PAGE_SIZE - DELTA, where
DELTA is a defined constant.   Callers that require an allocation
larger than that MUST use the page_alloc (and corresponding
page_free) interfaces.  In other words, for any dynamic allocation
code that needs a dynamically computed size that might exceed a
page, the test must be done on the caller-side... and the caller
is responsible for remembering whether the subpage allocator or
the page-plus allocator was used, and free''ing with the matching
subpage-free or page-plus-free routine.  While I''d never propose
this unforgiving API for user-land code, I think it isn''t unreasonable
in a hypervisor.

One compromise might be to retain the existing interface for
xmalloc_array() as calls to that routine are much more likely
to exceed a page.

Another might be to retain the existing xmalloc interface...
and maybe even the existing engine... but supplement it with
a new xmalloc_subpage() interface and change caller-side code
over time to use the new interface(/engine).

Comments?  (on both the API proposal and the engine)

Thanks,
Dan


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Dan Magenheimer schreef:
> Xen hypervisor experts --
...
> Comments?  (on both the API proposal and the engine)
Is this engine capable of progressing to a memory de-duplication style?
Or is that outside the scope?


Stefan
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Yes.  Well, for basic page de-duplication it probably
is irrelevant, but for "advanced features" it will be
more capable because it will handle near-page-size
allocations much better.
> -----Original Message-----
> From: Stefan de Konink [mailto:stefan@konink.de]
> Sent: Saturday, October 11, 2008 4:12 PM
> To: Dan Magenheimer
> Cc: Xen-Devel (E-mail); Diwaker Gupta; nitingupta910@gmail.com; Kurt
> Hackel
> Subject: Re: [Xen-devel] [RFC] Replacing Xen''s xmalloc engine 
> and(?) API
> 
> 
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA512
> 
> Dan Magenheimer schreef:
> > Xen hypervisor experts --
> 
> ...
> 
> > Comments?  (on both the API proposal and the engine)
> 
> Is this engine capable of progressing to a memory 
> de-duplication style?
> Or is that outside the scope?
> 
> 
> Stefan
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> 
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> WVQAn0ViQ/B3HtXaZLj7vPM0CHKOXc8B
> =2OZG
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> 
>
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On 11/10/08 22:44, "Dan Magenheimer"
<dan.magenheimer@oracle.com> wrote:
> As a result, I''d like to propose a change to the xmalloc interface
> to make this issue more explicit:  I''d like to change
xmalloc/xfree
> to FAIL on allocation sizes greater than PAGE_SIZE - DELTA, where
> DELTA is a defined constant.   Callers that require an allocation
> larger than that MUST use the page_alloc (and corresponding
> page_free) interfaces.  In other words, for any dynamic allocation
> code that needs a dynamically computed size that might exceed a
> page, the test must be done on the caller-side... and the caller
> is responsible for remembering whether the subpage allocator or
> the page-plus allocator was used, and free''ing with the matching
> subpage-free or page-plus-free routine.  While I''d never propose
> this unforgiving API for user-land code, I think it isn''t
unreasonable
> in a hypervisor.
This sounds crazy to me. xmalloc() should work like malloc().

 -- Keir



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On 12/10/08 10:16, "Keir Fraser" <keir.fraser@eu.citrix.com>
wrote:
> On 11/10/08 22:44, "Dan Magenheimer"
<dan.magenheimer@oracle.com> wrote:
> 
>> As a result, I''d like to propose a change to the xmalloc
interface
>> to make this issue more explicit:  I''d like to change
xmalloc/xfree
>> to FAIL on allocation sizes greater than PAGE_SIZE - DELTA, where
>> DELTA is a defined constant.   Callers that require an allocation
>> larger than that MUST use the page_alloc (and corresponding
>> page_free) interfaces.  In other words, for any dynamic allocation
>> code that needs a dynamically computed size that might exceed a
>> page, the test must be done on the caller-side... and the caller
>> is responsible for remembering whether the subpage allocator or
>> the page-plus allocator was used, and free''ing with the
matching
>> subpage-free or page-plus-free routine.  While I''d never
propose
>> this unforgiving API for user-land code, I think it isn''t
unreasonable
>> in a hypervisor.
> 
> This sounds crazy to me. xmalloc() should work like malloc().
For example, why not take Linux''s SLUB allocator? The fact
it''s tried and
tested in a real-world environment not unlike our own is a big advantage to
my mind.

 -- Keir



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> > On 11/10/08 22:44, "Dan Magenheimer" 
> <dan.magenheimer@oracle.com> wrote:
> > 
> >> As a result, I''d like to propose a change to the xmalloc
interface
> >> to make this issue more explicit:  I''d like to change
xmalloc/xfree
> >> to FAIL on allocation sizes greater than PAGE_SIZE - DELTA, where
> >> DELTA is a defined constant.   Callers that require an allocation
> >> larger than that MUST use the page_alloc (and corresponding
> >> page_free) interfaces.  In other words, for any dynamic allocation
> >> code that needs a dynamically computed size that might exceed a
> >> page, the test must be done on the caller-side... and the caller
> >> is responsible for remembering whether the subpage allocator or
> >> the page-plus allocator was used, and free''ing with the
matching
> >> subpage-free or page-plus-free routine.  While I''d never
propose
> >> this unforgiving API for user-land code, I think it isn''t
> >> unreasonable in a hypervisor.
>
> This sounds crazy to me. xmalloc() should work like malloc().
While I agree completely in principle, abstractions can be costly.
In this case, the fact that xmalloc(PAGE_SIZE-1) actually uses
2*PAGE_SIZE of space (and fails even if there are lots of free
pages but no pair of contiguous pages) is an undocumented consequence
of the underlying implementation... which is not entirely
unreasonable in user-land but is IMHO questionable in the guts
of a hypervisor where memory shouldn''t be accidentally wasted.

To date, Xen hasn''t focused much on optimizing memory usage but
I think that will change over the next year or two.
 
> For example, why not take Linux''s SLUB allocator? The fact 
> it''s tried and
> tested in a real-world environment not unlike our own is a 
> big advantage to
> my mind.
The slub allocator is still quite a bit more complex than
xmalloc or TLSF and its not clear that that complexity will add
much value, or solve the worst-case issues when allocating
a huge number of different-sized near-page-size units.

TLSF is very similar to the existing xmalloc except that it has
better response time, MUCH better worst-case response time,
and better fragmentation characteristics.

Adding a slub allocator to Xen might be useful to solve some other
problems but it doesn''t help me (or future de-duplication code
I don''t think).

(BTW, my proposal above to change the API is orthogonal to this
discussion.)

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On 12/10/08 17:28, "Dan Magenheimer"
<dan.magenheimer@oracle.com> wrote:
>> This sounds crazy to me. xmalloc() should work like malloc().
> 
> While I agree completely in principle, abstractions can be costly.
> In this case, the fact that xmalloc(PAGE_SIZE-1) actually uses
> 2*PAGE_SIZE of space (and fails even if there are lots of free
> pages but no pair of contiguous pages) is an undocumented consequence
> of the underlying implementation... which is not entirely
> unreasonable in user-land but is IMHO questionable in the guts
> of a hypervisor where memory shouldn''t be accidentally wasted.
> 
> To date, Xen hasn''t focused much on optimizing memory usage but
> I think that will change over the next year or two.
This wastage should be empirically measured by instrumentation and then
optimisations made where worthwhile. That is somewhat orthogonal to the
issue of what represents a sensible interface to xmalloc(), except to say
that I would generally prefer a more sophisticated and efficient mechanism
behind a simple interface, rather than punt complexity into callers (by
making the costs hidden by the simple interface excessive and hence
unusable; or by complicating the interface with weird constraints).

My point in bringing up SLUB is that I assume it''s an allocator
designed to
work reasonably well across a range of allocation-request-size
distributions, including those containing requests of size
x-pages-minus-a-bit. I''d rather have a more complicated allocator than
a
more complicated xmalloc() interface.

 -- Keir



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> >> This sounds crazy to me. xmalloc() should work like malloc().
> > 
> > While I agree completely in principle, abstractions can be costly.
> > In this case, the fact that xmalloc(PAGE_SIZE-1) actually uses
> > 2*PAGE_SIZE of space (and fails even if there are lots of free
> > pages but no pair of contiguous pages) is an undocumented 
> consequence
> > of the underlying implementation... which is not entirely
> > unreasonable in user-land but is IMHO questionable in the guts
> > of a hypervisor where memory shouldn''t be accidentally
wasted.
> > 
> > To date, Xen hasn''t focused much on optimizing memory usage
but
> > I think that will change over the next year or two.
> 
> This wastage should be empirically measured by 
> instrumentation and then
> optimisations made where worthwhile. That is somewhat 
> orthogonal to the
> issue of what represents a sensible interface to xmalloc(), 
> except to say
> that I would generally prefer a more sophisticated and 
> efficient mechanism
> behind a simple interface, rather than punt complexity into 
> callers (by
> making the costs hidden by the simple interface excessive and hence
> unusable; or by complicating the interface with weird constraints).
Fair enough.  I will mimic the xmalloc API then.  However, I *would*
like to export (via #define in xmalloc.h or function call or something)
the definition of DELTA (e.g. the xmalloc space overhead) so my
caller-side code can avoid the wastage.  I never want to accidentally
xmalloc two pages when heap-alloc''ing one page will do.
> My point in bringing up SLUB is that I assume it''s an 
> allocator designed to
> work reasonably well across a range of allocation-request-size
> distributions, including those containing requests of size
> x-pages-minus-a-bit. I''d rather have a more complicated 
> allocator than a
> more complicated xmalloc() interface.
I''m no slab/slub expert but I think the interface only works
well with fixed-size objects and when several of the fixed-size
objects can be crammed into a single page.  I have a large set
of objects that are essentially random in size (but all less
than or equal to a page).

Dan

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On 12/10/08 18:31, "Dan Magenheimer"
<dan.magenheimer@oracle.com> wrote:
>> This wastage should be empirically measured by
>> instrumentation and then
>> optimisations made where worthwhile. That is somewhat
>> orthogonal to the
>> issue of what represents a sensible interface to xmalloc(),
>> except to say
>> that I would generally prefer a more sophisticated and
>> efficient mechanism
>> behind a simple interface, rather than punt complexity into
>> callers (by
>> making the costs hidden by the simple interface excessive and hence
>> unusable; or by complicating the interface with weird constraints).
> 
> Fair enough.  I will mimic the xmalloc API then.  However, I *would*
> like to export (via #define in xmalloc.h or function call or something)
> the definition of DELTA (e.g. the xmalloc space overhead) so my
> caller-side code can avoid the wastage.  I never want to accidentally
> xmalloc two pages when heap-alloc''ing one page will do.
A better xmalloc() implementation would allocate the necessary two pages
from alloc_heap_pages() and put the remaining just-less-than-a-page on its
free lists, rather than waste it. Or put the allocation metadata out-of-band
(e.g., in page_info, like SLUB) so that there is no DELTA.

*However*, exposing DELTA for those callers who care about it, given
limitations of current xmalloc() implementation, is a reasonable way to go
as far as I''m concerned.
>> My point in bringing up SLUB is that I assume it''s an
>> allocator designed to
>> work reasonably well across a range of allocation-request-size
>> distributions, including those containing requests of size
>> x-pages-minus-a-bit. I''d rather have a more complicated
>> allocator than a
>> more complicated xmalloc() interface.
> 
> I''m no slab/slub expert but I think the interface only works
> well with fixed-size objects and when several of the fixed-size
> objects can be crammed into a single page.  I have a large set
> of objects that are essentially random in size (but all less
> than or equal to a page).
Well, you''d end up using the power-of-two sized caches. And since SLUB
doesn''t put metadata in the data pages, the 4096-byte object cache will
serve up true single pages. But yes, this is an aspect I hadn''t fully
considered. I''m not wedded to use of SLUB, it was just my arguing
stick. :-)

 -- Keir



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On 12/10/08 18:44, "Nitin Gupta" <nitingupta910@gmail.com>
wrote:
>> I''m no slab/slub expert but I think the interface only works
>> well with fixed-size objects and when several of the fixed-size
>> objects can be crammed into a single page.  I have a large set
>> of objects that are essentially random in size (but all less
>> than or equal to a page).
>> 
> 
> Using slab for your use case, which requires lot of allocations for
> near page-sized objects, is really bad idea. For memory compression
> project (compcache) I had very similar requirement. So, I tested
> kmalloc which uses slabs of various sizes to allocate various sizes.
> As expected its space efficiency was horrible. At least for compcache
> workload it used ~40% more memory than TLSF. Please refer:
> http://code.google.com/p/compcache/wiki/AllocatorsComparison
Yes, I was just going to reply to Dan''s TLSF email to say that my
default
plan for improving xmalloc() was going to be to implement a sub-page buddy
allocator or to port SLUB from Linux. Noth of these place metadata in the
page-info structure, but both would end up allocating power-of-two-sized
data blocks for random allocations of xmalloc(). The overheads of this --
for example if many allocations are just over 2048 bytes for example --
could be pretty nasty. I think TLSF sounds quite an intriquing alternative.

 -- Keir



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Hi Nitin --

Excellent!  Your xvMalloc looks even better!  I''ll read the
code and try it (tomorrow).

Keir, is a "GNU Lesser GPL Version 3" license OK for Xen?

Thanks,
Dan
> -----Original Message-----
> From: Nitin Gupta [mailto:nitingupta910@gmail.com]
> Sent: Sunday, October 12, 2008 11:45 AM
> To: Dan Magenheimer
> Cc: Keir Fraser; Xen-Devel (E-mail); Diwaker Gupta; Kurt Hackel
> Subject: Re: [Xen-devel] [RFC] Replacing Xen''s xmalloc engine 
> and(?) API
> 
> 
> On Sun, Oct 12, 2008 at 11:01 PM, Dan Magenheimer
> <dan.magenheimer@oracle.com> wrote:
> >
> > I''m no slab/slub expert but I think the interface only works
> > well with fixed-size objects and when several of the fixed-size
> > objects can be crammed into a single page.  I have a large set
> > of objects that are essentially random in size (but all less
> > than or equal to a page).
> >
> 
> Using slab for your use case, which requires lot of allocations for
> near page-sized objects, is really bad idea. For memory compression
> project (compcache) I had very similar requirement. So, I tested
> kmalloc which uses slabs of various sizes to allocate various sizes.
> As expected its space efficiency was horrible. At least for compcache
> workload it used ~40% more memory than TLSF. Please refer:
> http://code.google.com/p/compcache/wiki/AllocatorsComparison
> 
> Regarding non-standard TLSF interface, it should be quite simple to
> have wrappers around TLSF to have standard malloc/free API.
> 
> Also I recently completed draft implementation of a new allocator -
> xvMalloc (based on TLSF) that is specifically suited for allocation
> behavior of compcache (I think "difference engine" project has
almost
> same requirements).
> http://code.google.com/p/compcache/wiki/xvMalloc
> 
> Nitin
>
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On 12/10/08 19:03, "Dan Magenheimer"
<dan.magenheimer@oracle.com> wrote:
> Excellent!  Your xvMalloc looks even better!  I''ll read the
> code and try it (tomorrow).
> 
> Keir, is a "GNU Lesser GPL Version 3" license OK for Xen?
Unfortunately I doubt it, since Xen is GPLv2 and we wouldn''t really be
linking against xvMalloc as a separate library (not least because it would
probably get tailored a bit to get checked into the Xen tree) so the
''Lesser'' license probably doesn''t apply. If Nitin
isn''t too attached to
GPLv3 it would certainly be better for us if it used GPLv2, like TLSF.

 -- Keir



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