Detect Almost Full FDS to Perform Garbage Collection

[Matt22]

I would like to perform FDS garbage collection only when it is close to being full. I can read the fds_stat to see the number of words used and the words freeable but what is the maximum number for this if I have 8 virtual pages, each 1kb in size. Is there overhead such as CRC that is not included in these stats or can I just add these two numbers together and get the 8kb in complete size I have allocated?

[Jared]

Hi,

The FDS module use records_stat() which is defined as:

// Retrieve statistics about dirty records on a page.
static void records_stat(uint16_t   page,
                         uint16_t * p_valid_records,
                         uint16_t * p_dirty_records,
                         uint16_t * p_freeable_words,
                         bool     * p_corruption)
{
    fds_header_t const *       p_header   = (fds_header_t*)(m_pages[page].p_addr + FDS_PAGE_TAG_SIZE);
    uint32_t     const * const p_page_end = (m_pages[page].p_addr + FDS_PAGE_SIZE);

    while (header_has_next(p_header, p_page_end))
    {
        switch (header_check(p_header, p_page_end))
        {
            case FDS_HEADER_DIRTY:
                *p_dirty_records  += 1;
                *p_freeable_words += FDS_HEADER_SIZE + p_header->length_words;
                p_header = header_jump(p_header);
                break;

            case FDS_HEADER_VALID:
                *p_valid_records += 1;
                p_header = header_jump(p_header);
                break;

            case FDS_HEADER_CORRUPT:
            {
                *p_dirty_records  += 1;
                *p_freeable_words += (p_page_end - (uint32_t*)p_header);
                *p_corruption      = true;
                // We can't continue on this page.
                return;
            }

            default:
                break;
        }
    }
}

Which shows that the freeable words is defined as: *p_freeable_words += FDS_HEADER_SIZE + p_header->length_words;

FDS_HEADER_SIZE includes the CRC(see storage format). 

Best regards

Jared 

[Matt22]

Thank you, one thing that I noticed is that i should only use the words_used, this includes the freeable words, and the first FDS page is used for garbage collection so I am using this to detect when we are 90% full.

#define GC_NUM_BYTES ((FDS_VIRTUAL_PAGES-1) * FDS_VIRTUAL_PAGE_SIZE)*.9

if (stats.corruption || stats.words_used > GC_NUM_BYTES) {
garbage_collect();
}