On Fri, Aug 07, 2009 at 09:07:53AM -0400, Theodore Ts'o wrote:
>
> I've got two questions about mballoc's stream allocation.
>
> First of all, in ext4_mb_regular_allocator(), I'm 99% sure this is a
> bug:
>
> /* if stream allocation is enabled, use global goal */
> size = ac->ac_o_ex.fe_logical + ac->ac_o_ex.fe_len;
> isize = i_size_read(ac->ac_inode) >> bsbits;
> if (size < isize)
> size = isize;
>
> if (size < sbi->s_mb_stream_request &&
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> (ac->ac_flags & EXT4_MB_HINT_DATA)) {
> /* TBD: may be hot point */
> spin_lock(&sbi->s_md_lock);
> ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> spin_unlock(&sbi->s_md_lock);
> }
>
> Shouldn't that be ">=", not "<". We want to use the values saved in
> sbi->s_mb_last_{group,start} only if we are doing a stream allocation,
> which would be true only if the file is *larger* than
> s_mb_stream_request, no?
>
>
> The second question I have is with regards to ext4_mb_use_best_found(),
> we set sbi->s_mb_last_{group,start} on any data allocation; shouldn't we
> only be setting those values only if we were doing a stream allocation
> in the first place?
>
> Otherwise, any kind of allocation will end up moving the global goal
> block for stream allocations; even if it is a small allocation in the
> middle of some block group caused by the flag EXT4_MB_HINT_NO_PREALLOC
> being set.
>
> Am I missing anything?
>
I guess we should be setting the sbi->s_mb_last_{group,start} only when doing
small file allocation. We want to make sure small file allocation always
use the goal block near to the previous small file allocation request. So
(size < sbi->s_mb_stream_request) is correct. But we should not be doing
sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
always.
For large file allocation we wanted the new blocks to closer to that files previous
allocated block which ext4_ext_find_goal return as the goal value. So for
large files the goal value passed should represent the correct value.
NOTE: I am still behind ext4 list mails due to other commitments. Will start looking
at the mails, but mostly would be slow in replies
-aneesh
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