On Monday, October 3, 2016 at 9:17:00 PM UTC-4, Dormando wrote:
>
>
>
> On Mon, 3 Oct 2016, yuantao peng wrote:
>
> > Hi, -- I am reading memcached source code and got a question on this
> function: do_slabs_alloc_chunked, it is called by do_slabs_alloc if the
> request size is larger than
> > the slabclass size. I am curious why we don't just move to a slabclass
> with larger size instead? Also, I am not sure I understand the way it
> calculated the number of chunks
> > needed:
>
> That's only used if it's already at the highest slab class. It chains
> multiple chunks of the highest possible class together.
>
> > int csize = p->size - sizeof(item_chunk);
> > unsigned int chunks_req = size / csize;
> > if (size % csize != 0)
> > chunks_req++;
> >
> >
> > later on, we store the first chunk in do_item_alloc as follows:
> >
> > if (it->it_flags & ITEM_CHUNKED) {
> > item_chunk *chunk = (item_chunk *) ITEM_data(it);
> >
> > chunk->next = (item_chunk *) it->h_next;
> > chunk->prev = 0;
> > chunk->head = it;
> > /* Need to chain back into the head's chunk */
> > chunk->next->prev = chunk;
> > chunk->size = chunk->next->size - ((char *)chunk - (char *)it);
> > chunk->used = 0;
> > assert(chunk->size > 0);
> > }
> >
> > That means the first item has an item_chunk header next to item header,
> then the csize calculation is wrong, isn't it? Say if csize equals 2,
> we will actually need 3
> > chunks, because the first chunk will have more meta data
> (item+item_chunk), am I missing something here?
>
> Wish I didn't have to do that, it is pretty confusing. I can double check
> but the size of the header is added and removed in the appropriate places
> and it's gotten quite a lot of testing.
>
> That particular section you're looking at is settign the initial chunk
> size to the potential size of the chunk minus the offset the current chunk
> is into the overall item the header is stored into. So that should take
> into account the entire rendered metadata.
>
>
It is indeed quite confusing. :) Say if the highest slab class have size
2000 bytes, and sizeof(item) and sizeof(item_chunk) are all 40 bytes, If
I call slab_alloc with ntotal=3920 bytes,
since csize = p->size - sizeof(item_chunk) , csize would be 2000-40=1960,
and chunks_req=3920/1960=2.
the following code in do_item_alloc will set the first chunk size to
(2000-40)=1960,
chunk->size = chunk->next->size - ((char *)chunk - (char *)it);
but the first chunk is set to start at the location ITEM_data(it) as
follows, which is after the struct item meta data, shouldn't the chunk
size here is actually smaller than 1960 because item_chunk meta data of the
first chunk will also consume some space?
item_chunk *chunk = (item_chunk *) ITEM_data(it);
> > thanks
> > yuantao
> >
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> >
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