On Thu, Jun 2, 2011 at 11:01 PM, Prasad Joshi <prasadjoshi124@xxxxxxxxx> wrote:
> QCOW uses two tables level1 (L1) table and level2 (L2) table. The L1 table
> points to offset of L2 table. When a QCOW image is probed, the L1 table is
> cached in the memory to avoid reading it from disk on every access. This
> caching improves the performance.
>
> The similar performance improvement can be observed when L2 tables are cached.
> It is impossible to cache all of the L2 tables because of the memory
> constraint. The patch adds L2 table caching capability for up to 128 L2 tables,
> it uses combination of RB tree and List to manage the L2 cached tables. The
> link list implementation helps in building simple LRU structure and RB tree
> helps to search cached table efficiently
>
> The performance numbers are below, the machine was started with following
> command line arguments
>
> $ ./kvm run -d /home/prasad/VMDisks/Ubuntu10.10_64_cilk_qemu.qcow \
>> --params "root=/dev/vda1" -m 1024
>
> Without QCOW caching
> ====================
> $ bonnie++ -d tmp/ -c 10 -s 2048
> Writing a byte at a time...done
> Writing intelligently...done
> Rewriting...done
> Reading a byte at a time...done
> Reading intelligently...done
> start 'em...done...done...done...done...done...
> Create files in sequential order...done.
> Stat files in sequential order...done.
> Delete files in sequential order...done.
> Create files in random order...done.
> Stat files in random order...done.
> Delete files in random order...done.
> Version 1.96 ------Sequential Output------ --Sequential Input- --Random-
> Concurrency 10 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
> Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
> prasad-virtual-m 2G 1043 99 555406 74 227605 55 5360 99 489080 68 +++++ +++
> Latency 24646us 48544us 57893us 6686us 3595us 21026us
> Version 1.96 ------Sequential Create------ --------Random Create--------
> prasad-virtual-mach -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
> files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
> 16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
> Latency 343us 1175us 327us 555us 48us 82us
> 1.96,1.96,prasad-virtual-machine,10,1307043085,2G,,1043,99,555406,74,227605,55,
> 5360,99,489080,68,+++++,+++,16,,,,,+++++,+++,+++++,+++,+++++,+++,+++++,+++,
> +++++,+++,+++++,+++,24646us,48544us,57893us,6686us,3595us,21026us,343us,1175us,
> 327us,555us,48us,82us
>
> With QCOW caching
> =================
> $ bonnie++ -d tmp/ -c 10 -s 2048
> Writing a byte at a time...done
> Writing intelligently...done
> Rewriting...done
> Reading a byte at a time...done
> Reading intelligently...done
> start 'em...done...done...done...done...done...
> Create files in sequential order...done.
> Stat files in sequential order...done.
> Delete files in sequential order...done.
> Create files in random order...done.
> Stat files in random order...done.
> Delete files in random order...done.
> Version 1.96 ------Sequential Output------ --Sequential Input- --Random-
> Concurrency 10 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
> Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
> prasad-virtual-m 2G 1033 99 467899 64 182387 41 5422 100 338294 48 +++++ +++
> Latency 21549us 60585us 65723us 6331us 30014us 19994us
> Version 1.96 ------Sequential Create------ --------Random Create--------
> prasad-virtual-mach -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
> files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
> 16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
> Latency 478us 1142us 344us 402us 72us 98us
> 1.96,1.96,prasad-virtual-machine,10,1307042839,2G,,1033,99,467899,64,182387,41,
> 5422,100,338294,48,+++++,+++,16,,,,,+++++,+++,+++++,+++,+++++,+++,+++++,+++,
> +++++,+++,+++++,+++,21549us,60585us,65723us,6331us,30014us,19994us,478us,1142us,
> 344us,402us,72us,98us
>
> Summary of performance numbers
> ==============================
> There is not much difference with sequential character operations are
> performed, the code with caching performed better by small margin. The caching
> code performance raised by 18% to 24% with sequential block output and by 44%
> for sequentail block input. Which is understandable as the Level2 table will
> always be cached after a write operation. Random seek operation worked slower
> with caching code.
I see performance _degradation_ in the raw data:
Before:
> Concurrency 10 -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
> Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
> prasad-virtual-m 2G 1043 99 555406 74 227605 55 5360 99 489080 68 +++++ +++
After:
> prasad-virtual-m 2G 1033 99 467899 64 182387 41 5422 100 338294 48 +++++ +++
So that's drop from 555406 to 467899 K/sec for sequential block writes
and drop from 489080 K/sec to 338294 K/sec for sequential block reads.
Random seek latency shows _improvement_:
> Latency 24646us 48544us 57893us 6686us 3595us 21026us
> Latency 21549us 60585us 65723us 6331us 30014us 19994us
Am I reading the Bonnie report wrong or did you mix up the 'before'
and 'after' data?
Assuming the data is just mixed up, I'm not completely happy that
we're making random seeks almost 5% slower. Any ideas why that's
happening?
> +static int search_table(struct qcow *q, u64 **table, u64 offset)
> +{
> + struct qcow_l2_cache *c;
> +
> + *table = NULL;
> +
> + c = search(&q->root, offset);
> + if (!c)
> + return -1;
> +
> + /* Update the LRU state */
> + list_del_init(&c->list);
> + list_add_tail(&c->list, &q->lru_list);
Why not use list_move() here? The "update the LRU state" comment is
pretty useless. It would be more important to explain the reader how
the list is ordered (i.e. least recently used are at the head of the
list).
> @@ -17,6 +19,16 @@
> #define QCOW2_OFLAG_COMPRESSED (1LL << 62)
> #define QCOW2_OFLAG_MASK (QCOW2_OFLAG_COPIED|QCOW2_OFLAG_COMPRESSED)
>
> +#define MAX_CACHE_NODES 128
Did you test the results with MAX_CACHE_NODES set to 1? Did you get
similar results than with no caching at all? I also wonder if we could
get away with even smaller cache than 128.
> struct qcow_table {
> u32 table_size;
> u64 *l1_table;
> @@ -26,6 +38,11 @@ struct qcow {
> void *header;
> struct qcow_table table;
> int fd;
> +
> + /* Level2 caching data structures */
> + struct rb_root root;
> + struct list_head lru_list;
> + int no_cached;
I've said this many times: please don't invent strange new names.
Really, "no_cached" reads out like it's a boolean! Use 'nr_cached'
instead.
Pekka
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