[U-Boot] [PATCH V3 1/3] drivers: block: add block device cache

[Tom Rini]

On Wed, Mar 30, 2016 at 08:36:21AM -0600, Stephen Warren wrote:
> On 03/28/2016 11:05 AM, Eric Nelson wrote:
> >Add a block device cache to speed up repeated reads of block devices by
> >various filesystems.
> >
> >This small amount of cache can dramatically speed up filesystem
> >operations by skipping repeated reads of common areas of a block
> >device (typically directory structures).
> >
> >This has shown to have some benefit on FAT filesystem operations of
> >loading a kernel and RAM disk, but more dramatic benefits on ext4
> >filesystems when the kernel and/or RAM disk are spread across
> >multiple extent header structures as described in commit fc0fc50.
> >
> >The cache is implemented through a minimal list (block_cache) maintained
> >in most-recently-used order and count of the current number of entries
> >(cache_count). It uses a maximum block count setting to prevent copies
> >of large block reads and an upper bound on the number of cached areas.
> >
> >The maximum number of entries in the cache defaults to 32 and the maximum
> >number of blocks per cache entry has a default of 2, which has shown to
> >produce the best results on testing of ext4 and FAT filesystems.
> >
> >The 'blkcache' command (enabled through CONFIG_CMD_BLOCK_CACHE) allows
> >changing these values and can be used to tune for a particular filesystem
> >layout.
[snip]
> >diff --git a/disk/part.c b/disk/part.c
> 
> >@@ -268,6 +268,8 @@ void part_init(struct blk_desc *dev_desc)
> >  	const int n_ents = ll_entry_count(struct part_driver, part_driver);
> >  	struct part_driver *entry;
> >
> >+	blkcache_invalidate(dev_desc->if_type, dev_desc->devnum);
> 
> Doesn't this invalidate the cache far too often? I expect that
> function is called for command the user executes from the
> command-line, whereas it'd be nice if the cache persisted across
> commands. I suppose this is a reasonable (and very safe) first
> implementation though, and saves having to go through each storage
> provider type and find out the right place to detect media changes.

My initial reaction here is that we should stay conservative and
invalidate the cache more often rather than too infrequent.  I mean,
what's the worst case here, an extra read? A few extra reads?  We want
to make sure we keep the complexity to functionality ratio right here,
if we make the recovery/flashing/factory cases a whole lot better but
are leaving 1 second of wall clock time on the table when we've just
gained a minute, we're OK.

> >diff --git a/drivers/block/blkcache.c b/drivers/block/blkcache.c
> 
> >+struct block_cache_node {
> >+	struct list_head lh;
> >+	int iftype;
> >+	int devnum;
> >+	lbaint_t start;
> >+	lbaint_t blkcnt;
> >+	unsigned long blksz;
> >+	char *cache;
> >+};
> >+
> >+static LIST_HEAD(block_cache);
> >+
> >+static struct block_cache_stats _stats = {
> >+	.max_blocks_per_entry = 2,
> >+	.max_entries = 32
> >+};
> 
> Now is a good time to mention another reason why I don't like using
> a dynamically allocated linked list for this: Memory fragmentation.
> By dynamically allocating the cache, we could easily run into a
> situation where the user runs a command that allocates memory and
> also adds to the block cache, then most of that memory gets freed
> when U-Boot returns to the command prompt, then the user runs the
> command again but it fails since it can't allocate the memory due to
> fragmentation of the heap. This is a real problem I've seen e.g.
> with the "ums" and "dfu" commands, since they might initialize the
> USB controller the first time they're run, which allocates some new
> memory. Statically allocation would avoid this.

That is a good point.  But how would you hit this?  The problem in
ums/dfu was that it was several megabytes, yes?  My quick read over the
code right now has me thinking this is something measured in kilobytes.

-- 
Tom
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