Re: [PATCHv6 4/6] zram: add multi stream functionality

[Minchan Kim <minchan@kernel.org>]

On Fri, Feb 21, 2014 at 02:50:41PM +0300, Sergey Senozhatsky wrote:
> This patch implements multi stream compression support.
> 
> Introduce struct zcomp_strm_multi and a set of functions to manage
> zcomp_strm stream access. zcomp_strm_multi has a list of idle zcomp_strm
> structs, spinlock to protect idle list and wait queue, making it possible
> to perform parallel compressions.
> 
> The following set of functions added:
> - zcomp_strm_multi_get()/zcomp_strm_multi_put()
>   get and put compression stream, implement required locking
> - zcomp_strm_multi_create()/zcomp_strm_multi_destroy()
>   create and destroy zcomp_strm_multi
> 
> zcomp ->strm_get() and ->strm_put() callbacks are set during initialisation
> to zcomp_strm_multi_get()/zcomp_strm_multi_put() correspondingly.
> 
> Each time zcomp issues a zcomp_strm_multi_get() call, the following set of
> operations performed:
> - spin lock strm_lock
> - if idle list is not empty, remove zcomp_strm from idle list, spin
>   unlock and return zcomp stream pointer to caller
> - if idle list is empty, current adds itself to wait queue. it will be
>   awaken by zcomp_strm_multi_put() caller.
> 
> zcomp_strm_multi_put():
> - spin lock strm_lock
> - add zcomp stream to idle list
> - spin unlock, wake up sleeper
> 
> Minchan Kim reported that spinlock-based locking scheme has demonstrated a
> severe perfomance regression for single compression stream case, comparing
> to mutex-based (https://lkml.org/lkml/2014/2/18/16)
> 
> base                      spinlock                    mutex
> 
> ==Initial write           ==Initial write             ==Initial  write
> records:  5               records:  5                 records:   5
> avg:      1642424.35      avg:      699610.40         avg:       1655583.71
> std:      39890.95(2.43%) std:      232014.19(33.16%) std:       52293.96
> max:      1690170.94      max:      1163473.45        max:       1697164.75
> min:      1568669.52      min:      573429.88         min:       1553410.23
> ==Rewrite                 ==Rewrite                   ==Rewrite
> records:  5               records:  5                 records:   5
> avg:      1611775.39      avg:      501406.64         avg:       1684419.11
> std:      17144.58(1.06%) std:      15354.41(3.06%)   std:       18367.42
> max:      1641800.95      max:      531356.78         max:       1706445.84
> min:      1593515.27      min:      488817.78         min:       1655335.73
> ==Random  write           ==Random  write             ==Random   write
> records:  5               records:  5                 records:   5
> avg:      1626318.29      avg:      497250.78         avg:       1695582.06
> std:      38550.23(2.37%) std:      1405.42(0.28%)    std:       9211.98
> max:      1665839.62      max:      498585.88         max:       1703808.22
> min:      1562141.21      min:      494526.45         min:       1677664.94
> ==Pwrite                  ==Pwrite                    ==Pwrite
> records:  5               records:  5                 records:   5
> avg:      1654641.25      avg:      581709.22         avg:       1641452.34
> std:      47202.59(2.85%) std:      9670.46(1.66%)    std:       38963.62
> max:      1740682.36      max:      591300.09         max:       1687387.69
> min:      1611436.34      min:      564324.38         min:       1570496.11
> 
> When only one compression stream available, mutex with spin on owner tends
> to perform much better than frequent wait_event()/wake_up(). This is why
> single stream implemented as a special case with mutex locking.

Side note:

It's true for x86 but it could be changed in embedded system like ARM
which is lower power compared to x86 and it also could depend on
compression algorithm speed so I am thinking we need adaptive locking
which sometime work with mutex sometime work with spinlock dynamically.
Anyway, it is further work and I should investigate more.

Most important thing is that this patchset wouldn't make any regression
if we use mutex for single stream as default so I'm okay now.

> 
> In order to compile this functionality ZRAM_MULTI_STREAM config option
> required to be set, which will be introduced later.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  drivers/block/zram/zcomp.c | 101 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 100 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
> index 9661226..41325ed 100644
> --- a/drivers/block/zram/zcomp.c
> +++ b/drivers/block/zram/zcomp.c
> @@ -12,9 +12,14 @@
>  #include <linux/slab.h>
>  #include <linux/wait.h>
>  #include <linux/sched.h>
> +#ifdef CONFIG_ZRAM_MULTI_STREAM
> +#include <linux/spinlock.h>
> +#endif

Do we really need to include spinlock.h?

>  
>  #include "zcomp.h"
>  
> +extern struct zcomp_backend zcomp_lzo;
> +
>  /*
>   * single zcomp_strm backend private part
>   */
> @@ -23,7 +28,18 @@ struct zcomp_strm_single {
>  	struct zcomp_strm *zstrm;
>  };
>  
> -extern struct zcomp_backend zcomp_lzo;
> +#ifdef CONFIG_ZRAM_MULTI_STREAM
> +/*
> + * multi zcomp_strm backend private part
> + */
> +struct zcomp_strm_multi {
> +	/* protect strm list */
> +	spinlock_t strm_lock;
> +	/* list of available strms */
> +	struct list_head idle_strm;
> +	wait_queue_head_t strm_wait;
> +};
> +#endif
>  
>  static struct zcomp_backend *find_backend(const char *compress)
>  {
> @@ -63,6 +79,89 @@ static struct zcomp_strm *zcomp_strm_alloc(struct zcomp *comp)
>  	return zstrm;
>  }
>  
> +#ifdef CONFIG_ZRAM_MULTI_STREAM
> +/*
> + * get existing idle zcomp_strm or wait until other process release
> + * (zcomp_strm_put()) one for us
> + */
> +static struct zcomp_strm *zcomp_strm_multi_get(struct zcomp *comp)
> +{
> +	struct zcomp_strm_multi *zp = comp->private;

How about comp->stream instead of private?

Other than that, Looks good to me.

> +	struct zcomp_strm *zstrm;
> +
> +	while (1) {
> +		spin_lock(&zp->strm_lock);
> +		if (list_empty(&zp->idle_strm)) {
> +			spin_unlock(&zp->strm_lock);
> +			wait_event(zp->strm_wait,
> +					!list_empty(&zp->idle_strm));
> +			continue;
> +		}
> +
> +		zstrm = list_entry(zp->idle_strm.next,
> +				struct zcomp_strm, list);
> +		list_del(&zstrm->list);
> +		spin_unlock(&zp->strm_lock);
> +		break;
> +	}
> +	return zstrm;
> +}
> +
> +/* add zcomp_strm back to idle list and wake up waiter */
> +static void zcomp_strm_multi_put(struct zcomp *comp, struct zcomp_strm *zstrm)
> +{
> +	struct zcomp_strm_multi *zp = comp->private;
> +
> +	spin_lock(&zp->strm_lock);
> +	list_add(&zstrm->list, &zp->idle_strm);
> +	spin_unlock(&zp->strm_lock);
> +
> +	wake_up(&zp->strm_wait);
> +}
> +
> +static void zcomp_strm_multi_destroy(struct zcomp *comp)
> +{
> +	struct zcomp_strm_multi *zp = comp->private;
> +	struct zcomp_strm *zstrm;
> +	while (!list_empty(&zp->idle_strm)) {
> +		zstrm = list_entry(zp->idle_strm.next,
> +				struct zcomp_strm, list);
> +		list_del(&zstrm->list);
> +		zcomp_strm_free(comp, zstrm);
> +	}
> +	kfree(zp);
> +}
> +
> +static int zcomp_strm_multi_create(struct zcomp *comp, int num_strm)
> +{
> +	int i;
> +	struct zcomp_strm *zstrm;
> +	struct zcomp_strm_multi *zp;
> +
> +	comp->destroy = zcomp_strm_multi_destroy;
> +	comp->strm_get = zcomp_strm_multi_get;
> +	comp->strm_put = zcomp_strm_multi_put;
> +	zp = kmalloc(sizeof(struct zcomp_strm_multi), GFP_KERNEL);
> +	comp->private = zp;
> +	if (!zp)
> +		return -ENOMEM;
> +
> +	spin_lock_init(&zp->strm_lock);
> +	INIT_LIST_HEAD(&zp->idle_strm);
> +	init_waitqueue_head(&zp->strm_wait);
> +
> +	for (i = 0; i < num_strm; i++) {
> +		zstrm = zcomp_strm_alloc(comp);
> +		if (!zstrm) {
> +			zcomp_strm_multi_destroy(comp);
> +			return -ENOMEM;
> +		}
> +		list_add(&zstrm->list, &zp->idle_strm);
> +	}
> +	return 0;
> +}
> +#endif
> +
>  static struct zcomp_strm *zcomp_strm_single_get(struct zcomp *comp)
>  {
>  	struct zcomp_strm_single *zp = comp->private;
> -- 
> 1.9.0.291.g027825b
> 
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-- 
Kind regards,
Minchan Kim