Re: Need to potentially watch stack usage for ext4 and AIO...

[Eric Sandeen <sandeen@redhat.com>]

Eric Sandeen wrote:
> Theodore Tso wrote:

...

>> I can see some things we can do to optimize stack usage; for example,
>> struct ext4_allocation_request is allocated on the stack, and the
>> structure was laid out without any regard to space wastage caused by
>> alignment requirements.  That won't help on x86 at all, but it will
>> help substantially on x86_64 (since x86_64 requires that 8 byte
>> variables must be 8-byte aligned, where as x86_64 only requires 4 byte
>> alignment, even for unsigned long long's).  But it's going have to be
>> a whole series of incremental improvements; I don't see any magic
>> bullet solution to our stack usage.
> 
> XFS forces gcc to not inline any static function; it's extreme, but
> maybe it'd help here too.

Giving a blanket noinline treatment to mballoc.c yields some significant
stack savings:

-ext4_mb_free_blocks 200
+ext4_mb_free_blocks 184

-ext4_mb_init_cache 232
+ext4_mb_init_cache 136

-ext4_mb_regular_allocator 232
+ext4_mb_regular_allocator 104

-ext4_mb_new_blocks 104
(drops below 100 bytes)

-Eric