static ngx_int_t
ngx_http_upstream_process_cache_control(ngx_http_request_t *r,
    ngx_table_elt_t *h, ngx_uint_t offset)
{
…
#if (NGX_HTTP_CACHE)
    {
    u_char     *p, *start, *last;
    ngx_int_t   n;

    if (u->conf->ignore_headers & NGX_HTTP_UPSTREAM_IGN_CACHE_CONTROL) {
        return NGX_OK;
    }

    if (r->cache == NULL) {
        return NGX_OK;
    }

    start = h->value.data;
    last = start + h->value.len;

    if (r->cache->valid_sec != 0 && u->headers_in.x_accel_expires != NULL) {
        goto extensions;
    }

    if (ngx_strlcasestrn(start, last, (u_char *) "no-cache", 8 - 1) != NULL
        || ngx_strlcasestrn(start, last, (u_char *) "no-store", 8 - 1) != NULL
        || ngx_strlcasestrn(start, last, (u_char *) "private", 7 - 1) != NULL)
    {
        u->headers_in.no_cache = 1;
        return NGX_OK;
    }

    p = ngx_strlcasestrn(start, last, (u_char *) "s-maxage=", 9 - 1);
    offset = 9;

    if (p == NULL) {
        p = ngx_strlcasestrn(start, last, (u_char *) "max-age=", 8 - 1);
        offset = 8;
    }

    if (p) {
        n = ngx_http_upstream_process_delta_seconds(p + offset, last);

        if (n == NGX_ERROR) {
            u->cacheable = 0;
            return NGX_OK;
        }

        if (n == 0) {
            u->headers_in.no_cache = 1;
            return NGX_OK;
        }

        r->cache->valid_sec = ngx_min((ngx_uint_t) ngx_time() + n,
                                      NGX_MAX_INT_T_VALUE);
        u->headers_in.expired = 0;
    }

Warm-up