Reorganise structs layout to improve mem padding

[kuba--]

Signed-off-by: kuba-- kuba@sourced.tech
Closes #436

This is a small thing, but maybe worth to consider (WDYT)?. As an example I re-organized couple structs to make memory alignment optimal - less work for GC.

Changes:

• Server size was 192, now 176
• squashReposIter size was 80, now 72
• squashRefIter size was size 112, now 104
• squashCommitFilesIter size was 120, now 112
• squashIndexCommitFilesIter size was 128, now 120

[mcarmonaa]

I didn't aware about this and I feel curious, what is the reasoning behind it? How does it work? grouping fields by type?

[kuba--]

@mcarmonaa - couple readings:

• https://dave.cheney.net/2015/10/09/padding-is-hard
• https://go101.org/article/memory-layout.html
• http://www.catb.org/esr/structure-packing/
• https://syslog.ravelin.com/go-and-memory-layout-6ef30c730d51

(or just by searching about go memory alignment), but briefly - memory aligns to 8 bytes (most likely but it depends on architecture), so if you layout your structure like:

type MyData struct {
	a bool
	b int64
	c int32
	d bool
	e string
}

because of 8bytes alignment, compiler will introduce memory padding, so it will look:

MyData.a bool: 0-1 (size 1, align 1)
padding: 1-8 (size 7, align 0) # <---------------------
MyData.b int64: 8-16 (size 8, align 8)
MyData.c int32: 16-20 (size 4, align 4)
MyData.d bool: 20-21 (size 1, align 1)
padding: 21-24 (size 3, align 0) # <--------------------
MyData.e string: 24-40 (size 16, align 8)

and thats why you'll loose 10 bytes.

But if you layout your structure for instance as follows:

type MyData struct {
	e string
	b int64
	c int32
	a bool
	d bool
}

you'll loose just 2 bytes (I think the last padding is OK):

MyData.e string: 0-16 (size 16, align 8)
MyData.b int64: 16-24 (size 8, align 8)
MyData.c int32: 24-28 (size 4, align 4)
MyData.a bool: 28-29 (size 1, align 1)
MyData.d bool: 29-30 (size 1, align 1)
padding: 30-32 (size 2, align 0) # <----------

Generally, the simplest thing what I do I just layout structure from the biggest type to the smallest.

[mcarmonaa]

Thank you very much @kuba-- for such a good explanation and the links! ✨

[ajnavarro]

This is changing the output of gitbase server --help command? If yes, could you change it on the documentation too please?

[kuba--]

@ajnavarro - done