Thread: Memory Alignment in Postgres

On Tue, Sep  9, 2014 at 11:08:05AM -0300, Arthur Silva wrote:
> I'm continuously studying Postgres codebase. Hopefully I'll be able to make
> some contributions in the future.
> 
> For now I'm intrigued about the extensive use of memory alignment. I'm sure
> there's some legacy and some architecture that requires it reasoning behind it.
> 
> That aside, since it wastes space (a lot of space in some cases) there must be
> a tipping point somewhere. I'm sure one can prove aligned access is faster in a
> micro-benchmark but I'm not sure it's the case in a DBMS like postgres,
> specially in the page/rows area.
> 
> Just for the sake of comparison Mysql COMPACT storage (default and recommended
> since 5.5) doesn't align data at all. Mysql NDB uses a fixed 4-byte alignment.
> Not sure about Oracle and others.
> 
> Is it worth the extra space in newer architectures (specially Intel)?
> Do you guys think this is something worth looking at?
> 
> I'm trying to messing with the *ALIGN macros but so far I wasn't able to get
> any conclusive results. My guess is that I'm missing something in the code or
> pg_bench doesn't stress the difference enough.

Postgres reads data block from disk and puts them in shared memory, then
the CPU accesses those values, like floats and integers, as though they
were in allocated memory, i.e. we make no adjustments to the data from
disk all the way to CPU.

I don't think anyone has measured the overhead of doing less alignment,
but I would be interested to see any test results produced.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + Everyone has their own god. +

On Tue, Sep 9, 2014 at 10:08 AM, Arthur Silva <arthurprs@gmail.com> wrote:
> I'm continuously studying Postgres codebase. Hopefully I'll be able to make
> some contributions in the future.
>
> For now I'm intrigued about the extensive use of memory alignment. I'm sure
> there's some legacy and some architecture that requires it reasoning behind
> it.
>
> That aside, since it wastes space (a lot of space in some cases) there must
> be a tipping point somewhere. I'm sure one can prove aligned access is
> faster in a micro-benchmark but I'm not sure it's the case in a DBMS like
> postgres, specially in the page/rows area.
>
> Just for the sake of comparison Mysql COMPACT storage (default and
> recommended since 5.5) doesn't align data at all. Mysql NDB uses a fixed
> 4-byte alignment. Not sure about Oracle and others.
>
> Is it worth the extra space in newer architectures (specially Intel)?
> Do you guys think this is something worth looking at?

Yes.  At least in my opinion, though, it's not a good project for a
beginner.  If you get your changes to take effect, you'll find that a
lot of things will break in places that are not easy to find or fix.
You're getting into really low-level areas of the system that get
touched infrequently and require a lot of expertise in how things work
today to adjust.

The idea I've had before is to try to reduce the widest alignment we
ever require from 8 bytes to 4 bytes.  That is, look for types with
typalign = 'd', and rewrite them to have typalign = 'i' by having them
use two 4-byte loads to load an eight-byte value.  In practice, I
think this would probably save a high percentage of what can be saved,
because 8-byte alignment implies a maximum of 7 bytes of wasted space,
while 4-byte alignment implies a maximum of 3 bytes of wasted space.
And it would probably be pretty cheap, too, because any type with less
than 8 byte alignment wouldn't be affected at all, and even those
types that were affected would only be slightly slowed down by doing
two loads instead of one.  In contrast, getting rid of alignment
requirements completely would save a little more space, but probably
at the cost of a lot more slowdown: any type with alignment
requirements would have to fetch the value byte-by-byte instead of
pulling the whole thing out at once.

But there are a couple of obvious problems with this idea, too, such as:

1. It's really complicated and a ton of work.
2. It would break pg_upgrade pretty darn badly unless we employed some
even-more-complex strategy to mitigate that.
3. The savings might not be enough to justify the effort.

It might be interesting for someone to develop a tool measuring the
number of bytes of alignment padding we lose per tuple or per page and
gather some statistics on it on various databases.  That would give us
some sense as to the possible savings.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Wed, Sep 10, 2014 at 11:43:52AM -0400, Robert Haas wrote:
> But there are a couple of obvious problems with this idea, too, such as:
> 
> 1. It's really complicated and a ton of work.
> 2. It would break pg_upgrade pretty darn badly unless we employed some
> even-more-complex strategy to mitigate that.
> 3. The savings might not be enough to justify the effort.
> 
> It might be interesting for someone to develop a tool measuring the
> number of bytes of alignment padding we lose per tuple or per page and
> gather some statistics on it on various databases.  That would give us
> some sense as to the possible savings.

And will we ever implement a logical attribute system so we can reorder
the stored attribtes to minimize wasted space?

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + Everyone has their own god. +

On Wed, Sep 10, 2014 at 4:29 PM, Bruce Momjian <bruce@momjian.us> wrote:
> On Wed, Sep 10, 2014 at 11:43:52AM -0400, Robert Haas wrote:
>> But there are a couple of obvious problems with this idea, too, such as:
>>
>> 1. It's really complicated and a ton of work.
>> 2. It would break pg_upgrade pretty darn badly unless we employed some
>> even-more-complex strategy to mitigate that.
>> 3. The savings might not be enough to justify the effort.
>>
>> It might be interesting for someone to develop a tool measuring the
>> number of bytes of alignment padding we lose per tuple or per page and
>> gather some statistics on it on various databases.  That would give us
>> some sense as to the possible savings.
>
> And will we ever implement a logical attribute system so we can reorder
> the stored attribtes to minimize wasted space?

You forgot to attach the patch.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Thu, Sep 11, 2014 at 9:32 AM, Arthur Silva <arthurprs@gmail.com> wrote:
> I thought all memory alignment was (or at least the bulk of it) handled
> using some codebase wide macros/settings, otherwise how could different
> parts of the code inter-op? Poking this area might suffice for some initial
> testing to check if it's worth any more attention.

Well, sure, but the issues aren't too simple.  For example, I think
there are cases where we rely on the alignment bytes being zero to
distinguish between an aligned value following and an unaligned
toasted value.  That stuff can make your head explode, or at least
mine.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On 2014-09-11 10:32:24 -0300, Arthur Silva wrote:
> Unaligned memory access received a lot attention in Intel post-Nehalen era.
> So it may very well pay off on Intel servers. You might find this blog post
> and it's comments/external-links interesting
> http://lemire.me/blog/archives/2012/05/31/data-alignment-for-speed-myth-or-reality/

FWIW, the reported results of imo pretty meaningless for postgres. It's
sequential access over larger amount of memory. I.e. a perfectly
prefetchable workload where it doesn't matter if superflous cachelines
are fetched because they're going to be needed next round anyway.

In many production workloads one of the most busy accesses to individual
datums is the binary search on individual pages during index
lookups. That's pretty much exactly the contrary to the above.

Not saying that it's not going to be a benefit in many scenarios, but
it's far from being as simple as saying that unaligned accesses on their
own aren't penalized anymore.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services

On Thu, Sep 11, 2014 at 8:32 AM, Arthur Silva <arthurprs@gmail.com> wrote:
>
> On Wed, Sep 10, 2014 at 12:43 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>
>> On Tue, Sep 9, 2014 at 10:08 AM, Arthur Silva <arthurprs@gmail.com> wrote:
>> > I'm continuously studying Postgres codebase. Hopefully I'll be able to
>> > make
>> > some contributions in the future.
>> >
>> > For now I'm intrigued about the extensive use of memory alignment. I'm
>> > sure
>> > there's some legacy and some architecture that requires it reasoning
>> > behind
>> > it.
>> >
>> > That aside, since it wastes space (a lot of space in some cases) there
>> > must
>> > be a tipping point somewhere. I'm sure one can prove aligned access is
>> > faster in a micro-benchmark but I'm not sure it's the case in a DBMS
>> > like
>> > postgres, specially in the page/rows area.
>> >
>> > Just for the sake of comparison Mysql COMPACT storage (default and
>> > recommended since 5.5) doesn't align data at all. Mysql NDB uses a fixed
>> > 4-byte alignment. Not sure about Oracle and others.
>> >
>> > Is it worth the extra space in newer architectures (specially Intel)?
>> > Do you guys think this is something worth looking at?
>>
>> Yes.  At least in my opinion, though, it's not a good project for a
>> beginner.  If you get your changes to take effect, you'll find that a
>> lot of things will break in places that are not easy to find or fix.
>> You're getting into really low-level areas of the system that get
>> touched infrequently and require a lot of expertise in how things work
>> today to adjust.
>
>
> I thought all memory alignment was (or at least the bulk of it) handled
> using some codebase wide macros/settings, otherwise how could different
> parts of the code inter-op? Poking this area might suffice for some initial
> testing to check if it's worth any more attention.
>
> Unaligned memory access received a lot attention in Intel post-Nehalen era.
> So it may very well pay off on Intel servers. You might find this blog post
> and it's comments/external-links interesting
> http://lemire.me/blog/archives/2012/05/31/data-alignment-for-speed-myth-or-reality/
>
> I'm a newbie in the codebase, so please let me know if I'm saying anything
> non-sense.

Be advised of the difficulties you are going to face here.  Assuming
for a second there is no reason not to go unaligned on Intel and there
are material benefits to justify the effort, that doesn't necessarily
hold for other platforms like arm/power.  Even though intel handles
the vast majority of installations it's not gonna fly to optimize for
that platform at the expense of others so there'd have to be some kind
of compile time setting to control alignment behavior. That being
said, if you could pull this off cleanly, it'd be pretty neat.

merlin

Merlin Moncure <mmoncure@gmail.com> writes:
> Be advised of the difficulties you are going to face here.  Assuming
> for a second there is no reason not to go unaligned on Intel and there
> are material benefits to justify the effort, that doesn't necessarily
> hold for other platforms like arm/power.

Note that on many (most?) non-Intel architectures, unaligned access is
simply not an option.  The chips themselves will throw SIGBUS or
equivalent if you try it.  Some kernels provide signal handlers that
emulate the unaligned access in software rather than killing the process;
but the performance consequences of hitting such traps more than very
occasionally would be catastrophic.

Even on Intel, I'd wonder what unaligned accesses do to atomicity
guarantees and suchlike.  This is not a big deal for row data storage,
but we'd have to be careful about it if we were to back off alignment
requirements for in-memory data structures such as latches and buffer
headers.

Another fun thing you'd need to deal with is ensuring that the C structs
we overlay onto catalog data rows still match up with the data layout
rules.

On the whole, I'm pretty darn skeptical that such an effort would repay
itself.  There are lots of more promising things to hack on.
        regards, tom lane

On 2014-09-11 11:39:12 -0400, Tom Lane wrote:
> Even on Intel, I'd wonder what unaligned accesses do to atomicity
> guarantees and suchlike.

They pretty much kill atomicity guarantees. Atomicity is guaranteed
while you're inside a cacheline, but not once you span them.

> This is not a big deal for row data storage,
> but we'd have to be careful about it if we were to back off alignment
> requirements for in-memory data structures such as latches and buffer
> headers.

Right. I don't think that's an option.

> Another fun thing you'd need to deal with is ensuring that the C structs
> we overlay onto catalog data rows still match up with the data layout
> rules.

Yea, this would require some nastyness in the bki generation, but it'd
probably doable to have different alignment for system catalogs.

> On the whole, I'm pretty darn skeptical that such an effort would repay
> itself.  There are lots of more promising things to hack on.

I have no desire to hack on it, but I can understand the desire to
reduce the space overhead...

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services

<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On Thu, Sep 11, 2014 at 12:39 PM, Tom Lane <span
dir="ltr"><<ahref="mailto:tgl@sss.pgh.pa.us" target="_blank">tgl@sss.pgh.pa.us</a>></span> wrote:<br
/><blockquoteclass="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex"><span>MerlinMoncure <<a href="mailto:mmoncure@gmail.com"
target="_blank">mmoncure@gmail.com</a>>writes:<br /> > Be advised of the difficulties you are going to face
here. Assuming<br /> > for a second there is no reason not to go unaligned on Intel and there<br /> > are
materialbenefits to justify the effort, that doesn't necessarily<br /> > hold for other platforms like arm/power.<br
/><br/></span>Note that on many (most?) non-Intel architectures, unaligned access is<br /> simply not an option.  The
chipsthemselves will throw SIGBUS or<br /> equivalent if you try it.  Some kernels provide signal handlers that<br />
emulatethe unaligned access in software rather than killing the process;<br /> but the performance consequences of
hittingsuch traps more than very<br /> occasionally would be catastrophic. <br /></blockquote><blockquote
class="gmail_quote"style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><br /> Even
onIntel, I'd wonder what unaligned accesses do to atomicity<br /> guarantees and suchlike.  This is not a big deal for
rowdata storage,<br /> but we'd have to be careful about it if we were to back off alignment<br /> requirements for
in-memorydata structures such as latches and buffer<br /> headers.<br /><br /> Another fun thing you'd need to deal
withis ensuring that the C structs<br /> we overlay onto catalog data rows still match up with the data layout<br />
rules.<br/><br /> On the whole, I'm pretty darn skeptical that such an effort would repay<br /> itself.  There are lots
ofmore promising things to hack on.<br /><br />                         regards, tom lane<br /></blockquote></div><br
/>IndeedI don't know any other architectures that this would be at an option. So if this ever moves forward it must be
turnedon at compile time for x86-64 only. I wonder how the Mysql handle their rows even on those architectures as their
storageformat is completely packed.<br /><br /></div><div class="gmail_extra">If we just reduced the alignment
requirementswhen laying out columns in the rows and indexes by reducing/removing padding -- typalign, it'd be enough
gainin my (humble) opinion.<br /><br /></div><div class="gmail_extra">If you think alignment is not an issue you can
seesaving everywhere, which is kinda insane...<br /></div><div class="gmail_extra"><br />I'm unsure how this equates in
patchcomplexity, but judging by the reactions so far I'm assuming a lot.<br /><br /></div></div> 

On Thu, Sep 11, 2014 at 02:54:36PM -0300, Arthur Silva wrote:
> Indeed I don't know any other architectures that this would be at an
> option. So if this ever moves forward it must be turned on at compile time
> for x86-64 only. I wonder how the Mysql handle their rows even on those
> architectures as their storage format is completely packed.
> 
> If we just reduced the alignment requirements when laying out columns in
> the rows and indexes by reducing/removing padding -- typalign, it'd be
> enough gain in my (humble) opinion.
> 
> If you think alignment is not an issue you can see saving everywhere, which
> is kinda insane...
> 
> I'm unsure how this equates in patch complexity, but judging by the
> reactions so far I'm assuming a lot.

If the column order in the table was independent of the physical layout,
it would be possible to order columns to reduce the padding needed. Not
my suggestion, just repeating a valid comment from earlier in the thread.

Regards,
Ken